GOBLIN TOWER
When you exit the Inner Sanctum tower after your talk with Lady Stadi Galanos, or later (after you've done the quest for the Amulet of the Righteous for instance, or after a visit to the city or to Vvardenfell), Adreian will (still) be waiting for you. Talk to him and he'll have an urgent demand: you need to free the Inquisitor from the goblins! Go to the port together and walk (in a normal pace - Adreian is only an ordinary Ordinator, and maybe not as quick as you; and make sure he keeps following you and doesn't dive into the water or something) towards the beacon-light, near where you first arrived - that's where Dark Wolf's ship will be waiting for you.
Dark Wolf is in his cabin, of course. He'll take you to the Bitter Coast, where an Ordinator Captain is waiting for you. Talk to him immediately. You're told that his Ordinators will fight a group of goblins, and that you should make use of this to rush through to go to the tower. You're told not to help them, so hurry through between the fighters or go around them (maybe you prefer to remain unseen, if that's your style); or ignore the Captain's orders and join in the fight against the goblins anyway (if you're a Patriarch of the Temple, that's the right thing to do - stand by your men - but you don't have to do that at all). But if you want to avoid the fight you have to hurry, or the goblins will be after you!
On your left you will see the tower. You need to go there, but run on until you're clear of the fight and see a boardwalk to the left. Take that path to the first little island, and look around from there. To the north you'll see a daedric ruin, Addadshashanamma (try saying that three times in a row
); to the west is the goblin's base of operations, Goblin Tower, with two very big goblins standing on the rocks next to it; and to the southwest the battle between the goblins and the Ordinators should still be going on, if you hadn't joined them.
There are two ways to approach the tower: openly or sneakily. If you show yourself to the giant goblins, they will throw heavy rocks at you, and they're very strong, so this is only recommended for strong characters, or skilled archers. But if you do this and defeat them, you can go and try the front door. The Ordinator Captain warned you that this way (the obvious way in) might be trapped, and he's right.
So it's better to go around the back to look for another way in, and there it is: an underwater cavern door. A short way into the cave there's a fake door - you'll only be greeted by an orc on the other side. Walk on and enter the door at the end - there he is. Kill him and look around: there doesn't seem to be another door, but touch the walls and you'll discover a hidden door (get used to this, you'll see more of these doors) to the tower proper.
Go downstairs and go through the first door you see, into the room: it's a kind of barracks for the goblins. Most of them are dead on the floor, however. The Inquisitor you're looking for isn't here; the prison is further downstairs, behind another hidden door. That door is locked, so you need to find a key inside the barracks first. In the room at the far end (watch out for the Durzog there, it's not very docile) there's a key on the wall amongst the knives, but that one is useless. The key you need is in one of the cells that the goblins sleep in. When you exit the room with the Durzog, go right. Behind the second door on the left is a goblin, the key is on the crate next to the candle.
Don't forget to search the goblins' dead bodies for health potions, and make a habit of that; goblins almost always carry one, and you're going to need them later on.
Go downstairs and through the hidden door into the prison, and talk to Gandor Amsirva. (You don't need to worry about opening the cell door btw.) He'll ask you to hurry to Gnaar Mok, to the Black Mill there (you're finally going to see it!) to kill Hasib, because according to the Inquisitor he's working for the enemy. The story may become a little confusing here...
So you go to Gnaar Mok, the Black Mill is west of the town. The shortest way there is to waterwalk straight to the north. An orc will receive you, but he's easy to defeat. The mill itself doesn't seem to have an entrance, but there's a submerged cavern door on the north side of the island that's easily visible. Pick up the potions inside (they might be useful sometime in the future, especially "who") and proceed. Hasib is waiting and will talk to you, but you insist on fighting him. Just follow the story from there. First Tommy and then the Inquisitor will enter the scene, and they'll both try to persuade you to their side. Eventually you're left with four choices, but only one of them is a viable option. Who's fake and who's real?
It may not be a clear-cut decision to you, but trust me, it's best to apologize to Hasib. Sure, Hasib attacked you first, but let's be honest, you were on your way to kill him, weren't you? Of course he would defend himself. The last answer is just naive, and the first two are more or less the same.
After the battle with the false Inquisitor, Hasib will ask you to go to further into the cave together, so he can disappear into the Last City, and you can discover the location of the four Shrines of the Elements. But you cannot enter them yet, this walk is only meant to let you see where the entrances are. Down in the caves you'll be warmly received by goblin forces (watch out for stepping stones too, in the first cave), especially when you explore further down and go looking for the entrance to the Shrine of Terra, or Terra Tenebrae. Since there's nothing else left to do for you there, you can leave the caves and do one of the following things, if you haven't done them already:
- Find Timpy in Sadrith Mora
- Find Myth in Ebonheart (if you started the quest for opening the transport route to Ebonheart
- Go to the Ebony Mill in Caldera and enter the Realm of Terra
- Go to the other Realms, as per Timpy's directions: the Realm of Aqua, the Realm of Aer, or the Realm of Ignis.
There's no set order to do these things, so go ahead and make your own choices.
You could stumble upon a mill by accident, but it's easier to find Timpy first and let him tell you where they are.
Btw, if you're in the upper caves, Almsivi Intervention will get you to Balmora quickly.