The false Lord Keto is located at X: 61 Y: 7. If your allegiance is with his wife, this is where you start, presumably by killing him. Otherwise, you'll have to come back here later to kill him. His throne is at X: 61 Y: 4. It's a classical "Damius-throne", ie if you sit in it you might be immortalized in the shape of a statue…
There's a secret passage at X: 49 Y: 2. Behind it are some troops waiting. After having killed them, continue to X: 43 Y: 6 and throw a knock spell at the door. Then go to X: 43 Y: 8. There's a dead warrior there, with some nice equipment. There's also a slight chance that you'll find a letter that he was to take from Lord Keto to Zukar at X: 46 Y: 10.
When you have returned to the throne chamber, go to X: 67 Y: 2. There you will be teleported to "somewhere else". Go to X: 63 Y: 31 and you will find a map with vital information regarding the construction of the two castles. You can also reach the room to which you are teleported by using a secret passage located at X: 71 Y: 33.
At X: 75 Y: 21 is a secret passage. Walk up the corridor and in through the door to find the real Lord Keto.
At X: 58 Y: 20 there is a couple of scrolls.
If you touch the experiment at X: 55 Y: 30, some mages will attack you.
At X: 53 Y: 24 you'll find a robe. Every time you walk onto that square you'll find the robe. It's just a plain Robe of Vulnerability.
There are some magic users practising in the room at X: 48 Y: 24. Kill them, then continue to X: 41 Y: 22. Beyond that secret passage you'll find Ulmac with some friends.
At X: 26 Y: 15 are some magical items.
At X: 46 Y: 34 is a hole that only one party member can enter. Pick a strong one, kill the enemy, and pick up the treasure.
At X: 45 Y: 52 you'll here some strange noices from the surrounding rooms. Well, that's because there are wild animals in those rooms.
There's a false floor at X: 23 Y: 37.
A secret passage at X: 27 Y: 55. And some treasure at X: 27 Y: 55.
At X: 45 Y: 42 is the exit from the castle.
There is a secret passage to Lady Keto's castle at X: 45 Y: 58.