I love that fort. I came across it relatively recently, I have played Skyrim for a few years but each new character seems to find something new.

Lake Geir

She found Narfi across the swift flowing waters from Ivarstead. Looking at the decrepit house, and its owner, brought back back the uneasy feeling in her stomach from the time of her stay above Lake Ilinata. She wanted her own nest yes, but not where a Nord's taint lingered, even after death. Still, she'd started him talking so let the old one finish.
She found the lost sister, by accident as she was chasing breakfast in the waters near the bridge. A few little treasures with the skeleton, and something for Narfi to remember his dead by.
The innkeeper told her of an alchemist towards a pass to Helgen in Falkreath. Uneasy away from the lake, she was at first wary as she made her way under the shadow of the Mountain. A stream of bees attracted her attention, then she saw a nest and another, and another. She remembered her caution when the last one was at the mouth of a bear cave.
More watchful now, feeling vulnerable to what the land could throw at her, she took to her heels at the feel of poison on her scales. Ran faster when she heard a huff and a roar on her left as she fled down hill to the safety of the waters. By Hist, that alchemist could keep their work to themselves.
At the worst, she could travel to the city of Riften. Were her kind allowed to pass the gates there? Memories mostly shut away reminded her of the old Saxheel who used to live there.

The days passed, Wilhelm was one of those rare Nords who made her welcome, and the other villagers just lived their own lives. And the waters were so close.
She did investigate the cave on the island. It was home to a bagful of mushrooms. Her collection of dried ingredients grew.
The story of the local ghost niggled at her. Weeks distant from the experience, she felt a growing shame at running from that vengeful sword waving ghost in the fort of the falls. Not that she would have braved the sword, but maybe she could have tested her poisons for one that touched a ghost. The sword felt real enough. Would it hurt to look?
She felt safer in the night. Then she found it asleep! She stabbed it with the old woman's poison and it stilled for long moments as she stabbed and stabbed wiping oils of all shades of green on her old blade, then running and sending an arrow coated with the deadliest poison and then she could stop running! She killed....a ghost that died as a man.
She read his journal and understood. She looked around the room at the alchemy equipment and furnishings. She studied the corpse, thinking of which side room she could dump it in. Then she read the journal again, and left the room and returned to the inn. It semed that Riften might be a wiser choice for an alchemy lab.

Wilhelm gave her a jewelled claw. The dead ghost's journal had showed her what this was. Emboldened by her success against a ghost, she returned to the barrow.
She checked on the corpse, yes it was still dead, and smelt dead too.
She found her way to the door for the claw, through a carved hall empty of threats. She stared at the claw for a long time, her brain slowly understanding, her guts slowly tying themselves in knots. She moved the stone carvings in place, inserted the claw and turned. Slowly the door sunk.
From the far end of the hall she'd retreated too, she stared towards the opening. Step by step, she returned to the sunken door, then passed through the portal, then crept down the stairs that spiraled down the stone.
Into a chamber that smelt of old death and promised riches. Coffins. Echoes. With lids on. No treasure. A book on a pedestal. Got it. Not a book, a tome of a spell...maybe one day she'd know enough to open it. She was more than halfway through the chamber....not far from the arched way...
Trapped! With a clang that woke the dead. Literally roused two. Running and firing with arrows of plain iron because everybody knows that alchemical oils made of the stuff of Nirn don't touch the woken undead. Just hiding and firing and releasing her small fountain of healing and then the blessing of her Histskin.
One fell, bristling with arrows, but then there were more than the two, every last coffin woke and her arrows were scattered all over the chamber. Enough in the undead that they were finally still.
Drained past fear, she collected the arrows one by one, kicked over an urn but it was empty, trudged back up the winding wooden steps and found at last the safety and comfort as the deep water of the lake closed over her.
She vowed she wouldn't return to those places until those streams within her were strong enough to reach past her own healing and touch the undead with the 'poison' of life.

Riften

She took her time, swimming slowly under the piers and following the wall. She liked this, a city extending over the lake. She wondered if trapdoors allowed one to drop straight into the waters. Did the argonians live in the fisheries?
As she approached the wooden gates she wondered if they'd let her in? Feeling more unsure with each step, she wondered if her grand-sire had lived inside the city.
"Before I let you into Riften, you need to pay the visitors tax."
Perking up she thought, "....let me in....he Will let me in."
The Nord guard watched every move as she fumbled to retrieve her pouch from the bottom of her pack. Her gaze shifted to meet his as she pulled out a handful of coins. 149 gold, more than half of her savings from Solstheim.
So, in fact, they only have rich argonians living in the city. If any. Never the less, it was too late to back out now. The gold was paid. The gate was opened.

Shoulders back, head high, she attempted to look as if she belonged, though inside she was frantically looking for signs of kin.
"Maybe they are all in the warehouses at the docks. Maybe the fishers are all that's here".
Then she saw him at his stall. She stood still, watching this argonian, this Saxheel, carrying on his business in this City. When a burly nord shoved her out of his way, she moved, and went to meet Madesi.
It turned out Madesi was indeed her grand-sires friend. It was to him they had been travelling on that dreadful day. Had it happened? Had it been her?
She stood beside him for the rest of the afternoon. When she explained who she was now, what she was, he made a request of her. She was eager to give him a gold nugget found at the base of a cliff, a lot of waters flowed since then. She would have given him all her small supply, but she firmly told herself she wasn't a hatchling that tries to buy favour or affection or belonging, she was an adult who knew worth.
He didn't invite her home with him, he told her to go to the Bee and Barb. That was all right, she was a stranger passing through.
Another pleasant surprise at the inn, Saxheel owned it. The feeling was growing, that this was her home.

She tracked down the alchemists, they were pleasantly situated nearly at water level. She was allowed to use their equipment, and they didn't mind her asking questions. Many hours' work went into transforming her catchings and gatherings into a growing stash of potions. Keerava had allowed her to keep a sack in the storeroom for her gear. She reminded Glimmers of her Aunty back on the Docks.

Level 11 Alchemy 30 Experimenter. Yay! Now she can really start on her potion studies.
                                  Poisoner

She had been away from Riften overnight, ending an excursion the length of the two lakes back at Ivarstead. It was dark the following evening when she returned to Lake Honrich, and caught glimpses of torch bugs and lunar moths in a lakeside copse. Something new to take back and study.
She slept late the next day, and it was late afternoon when she emerged from the alchemist's shop. She would venture along the other shore tonight.
As she wandered between shore line and rocky ridges, she felt an insistent internal tug. In contemplation as she paced , it came to her the feeling was familiar. A few more paces, and she remembered the Stone on an island on a different lake. She turned and stared up at the rock face.
Much navigation of rocks later, she found her way up to the ring of stone and found a vantage point from where she could look down. A mage was there, as she watched, they released a fiery figure onto an unfortunate fox.
She stayed in the shadows of the cliffs and the trees, thinking. This wasn't an undead. She had brewed all sorts of oils this afternoon to attack a living body by all sorts of routes. And Hafjorg had assured her she had made a couple of potions to make her appear not there. In case of need.
The fiery figure dissipated. Glimmers prepared some arrows, shifted away from the cliff to get a clearer view from behind a tree and released her arrows. The robed figure was affected, but still kept coming, magick charged and fresh atronach conjured as Glimmers fumbled to find her dagger.
She blinked and stared as the mage dropped still some distance away, A bubble of joy burst inside her. That was her poison did that, the life leeching one. She felt...significant. Even without the stone, this would still be a night for her to celebrate.
She confronted the Stone and laid her hand against it and the Shadow made Itself part of her.

She was looking unsuccessfully for the lights of insects near the trees surrounding the Talos shrine when she heard shouting and sounds of fighting. Keeping to the shadows, she found the soldiers fighting figures, fighting vampires in the town square. She stayed low , until the vampire had been cut down. Townspeople had been slaughtered, not Madesi thank goodness, he wasn't a night wanderer. Old habits die hard, and the soldiers, apart from the one dead, were on the far side of the square. She found a couple of keys and some jewelry on two smoothskin corpses near her. She took them and scarpered. That's as much as she did, here in Riften, she was too scared of Veeraza to do more.
She had discovered that Madesi slept in a hovel below the town. Tonight she had been going to join him there. It was no worse than Windhelm after all, and he ....well he was precious to her like her grand-sire had been. Now, after the vampires so close, she just wanted to take refuge in Veeraza's clean upstairs room.

Shadow Stone. Like her own wandering sign, she picks up any sign she comes across, until she finds the Serpent. ( I avoid the levelling and ritual stones)

Dark waters

It was near dusk when she left Riften for new waters. Her day had been spent at Elgrim's , working through her stock of Rift ingredients and experimenting with techniques of purification and distillation of her poisons.

Level 13 Alchemy 40 Bottomless Cup: Poisons applied to weapons last for 1 extra hit/ 10 ranks alchemy.

She swam leisurely towards Ivarstead, keeping an eye out for lighted insect in the trees. She was late arriving for the night, and late in the morning leaving.
She allowed the swift waters to carry her past the town, until she heard the roar of the troll close to the edge of the river. She scrambled and tumbled down rocks vainly seeking for a boulder to hide behind. A goat saved her, distracting the monster and giving her moments to widen the gap and find a track leading away.
She rejoined the waters when she found a small pool between cataracts. She allowed herself time to relax, to look around and find some mushrooms growing in the shade. Chasing a fish, she drew too close to the lip of the pool and the falling waters swept her down to a larger expanse of water spreading out between stone cliffs.
There she saw a carvings flanking a portal, opening into the sheer rockface itself. What was in the depths of the mountain? Her springs of healing rising, her arrows all tainted, she ventured through the door.
The air was drenched with moisture. She could sense life grown in the dark and the damp, life and death that cycled out of reach of the light of Magnus. This life provided its own phosphoresence instead. In the damp air, she could sense and smell the presence of the sentient creatures that dwelt in these depths of stone and water.
The carved stone tunnel widened into a platform overlooking a deep cavern. There she saw the first inhabitant prowling along a wooden extenstion of the overhang. It took her three arrows for her poisons to down the gaunt figure. The first of those that lurked in the shadows and high in stone cavities. But she could lurk and hide in the dark too, and kill in silence. Their home could be hers.
The way led down to the pool fed by water gushing from high on the rock face, the same waters as the great falls she'd travelled beside. She peered over the edge of the basin where water continued to fall. Far below was a dark lake and she thought she caught sight of another figure. On her level was a ramp and a tunnel leading further into the rock.
She collected arrows from the quiver of a dead one, she had few of her own left. She moved almost noiselessly into the shadows, and the tunnels took her to a pair of chambers. More hide and seek with the inhabitants, they never caught a glimpse of her as her arrows implanted their poisons. As she stalked and was stalked, she became aware of a fighting at a lower level, and a voice of a surface dweller.
When there seemed no more, she remained still in the shadows, watching and waiting. No more came, so she left the sides of the chamber to stare down past the grated floor. There, there in the stone was a pit with one dead, such as she'd been slaying, and ....and a kin!
She searched for, and eventually found the opening to the stairs spiralling down, while always aware ominous chittering coming up from the dark water to which the tunnel on the far side of the chamber led.
Derkeethus from Darkwater Crossing! Of all the treasures, of all the beings found in deep places, a lost and imprisoned saxheel armed with a pick axe was something outside any expectations. It was now she was truly afraid, that she would fail to bring this miner back to the surface waters. But she did, and they parted by the moonlit pool. She spent the rest of the night there, under the wide star lit sky.

Level 14 Restoration 30 Edgewalker: Restoration spells are up to 30% more powerful  if recipient is below half health

I'm glad it's not just my character that sneaks into a cave, has a look, and backs out quick. Well, at least Ostium  got what he wanted  needed eventually.

Ice

Of course she took him with her. The lost Saxheel from Darkwater crossing. How few of her kin did she find wandering alone over (and under) Skyrim. If he was ready for some more swimming into dark places, she was ready for him.
They slept at Mixwater Mill, and spent the days amongst the steaming astringent Eastmarch pools. They collected bright dragontongues and sprawling creepers and clusters of grapes. They entered a cave and marvelled at a ancient tree, its roots bound only to Skyrim. They braved skeletons and bears and managed to stay alive while they scavenged for gear for Derkeethus and arrows for them both.
They found the Atronach stone and Glimmers left the Shadows for Magick.

Then downstream they travelled, into the frigid waters that flowed below Windhelm. There was nothing at the Docks for them, any hope there was built on lies. Better skooma than that. They shared their salmon with Glimmers' close-kin and left.
Under and between ice, they followed the river's flow. Derkeethus followed her into an icebound barrow following clues and tunnels until they entered a hall carved with storied figures, ending in an arched portal of moving stones, opened with the stone claw. He could smell the fear on her. Then they ran for their lives up the winding passages and out through the ice to the sanctuary of the waters.
As the river widened and found the Sea, they came across a wrecked ship held between rock and ice. A shelter from the unending winter.
Over the next few days they ranged north, until there was no land, no rock or sand, only ice and water. They battled a sabre cat and won, only because water was near. Glimmers found a crate caste on a strip of gravel at the edge of an island of ice. It held precious gold ingots, for her future. She found a nirnroot, growing in ice.
They entered a crevice in the ice, found atronachs which they bettered, Glimmers holding firm within her fountains of healing as Derkeethus hacked from behind. In another cave, they found trolls, and fled.
They came ashore at Winterhold and basked in the warmth of the tavern. They scrounged up coin from selling potions in the village and visited the College, seeking knowledge. Glimmers gained an edge to her waters of healing, learnt how to make them Hurt.

Restoration 40 False light: Healing spells cast on enemy damage 75% of heal amount.
Healing hands spell

They spent more time amongst the ice, and came across another wreck claimed by smoothskin bandits. In bringing down the armoured leader she learnt a new way to kill, drawing on ancient Marsh memories to taint her flow of healing magick as poison corrupts the sweet scent of nightshade.

Level 15 Restoration 40 Necromanticon: Putrefy spell (1/3)

They found a journal amongst the crates. The boat was the Tel Vos and the journal held the secret long sought by her mer friend, marsh-named Brand-sei. From her 'home" city.
On a high island she caught up at last with the Stone of her birthsign, Serpent. Surprisingly, the ice wraiths were not brought down by the sharp false edge of her healing, but by her Marsh plague.
In a cavern, deep in the ice they found a crazy wise man and learnt of the Deepest of the Deep,that would one day be her journeys end,or the end of her journeys.      Blackreach.

"Under deep. Below the dark. The hidden keep."