Durlag's Tower Upper Floors Walkthrough - Baldur's Gate 1
The upper floors of Durlag's Tower include all 4 of the tower's upper floors and the first floor that we entered on as well as the cellar beneath. You could call this part of the dungeon the preamble to the real dungeon which is the 4 labyrinth levels that consist of the lower floors. For the most part there is not much valuable loot to be found on the upper floors, just some lore about Durlag's Tower and a lot of EXP from all the enemies and the one side quest.
Below I have detailed all you can find in the upper floors in an easier to read format. If you'd like to see some map locations they're provided further down the page as well.
(Tower) 3rd floor loot: Tome of Understanding north of Riggilo
(Tower) Outside portion: Basilisks as well as Rashad's Talon +2
Kirinhale & Riggilo: You will meet a succubus on the 4th floor by the name of Kirinhale who wants to leave this tower. Offer to help her and you will be given a lock of her hair which you can take to Riggilo on the floor below you. Riggilo won't accept the hair from you unless you tell him you're giving it to him to avoid a dagger in the back. After giving Riggilo the hair return to Kirinhale and talk to her for 4000 EXP.