All 4 floors have fixed layouts. The tower is circular. The floor numbers shown on the minimap are in reverse order, so the player will start on the 4th floor underground, and must climb up to fight Tannen on the 1st floor.
The basement. Divided into four chambers, with a corridor going round the outside of the floor, and a cross shape of corridors going through the middle, in between the chambers. The staircase up is in the middle. You begin in the south-west chamber, with a note from Tannen called "Welcome to your cell". To reach the staircase up, you need to visit all four chambers and use the lever in each, to open the doors in the middle.
This floor is largely covered in water, and has only aquatic enemies. You come up in the centre of the floor, in the middle of a spiral. The lore "Personal note (1)" is in the spiral. After exiting the spiral, you will be on the south-west side of a corridor around the outside of the level. The staircase up is on the north-east side of the corridor.
This floor has four chambers, a centre chamber, and a corridor around them. You come up in the north-east side of the corridor. To the south-west, you will find a door into one of the chambers, and you need to proceed anticlockwise through the four chambers to reach the centre chamber with the up staircase. The lore "Personal note (2)" is in the north-west chamber.
This floor is an open circular space. There is no staircase back down. You fight Tannen and his Drolem here. Upon picking up the Orb of Many Ways, Blood-Runed Athame, and Resonating Diamond that they drop, an exit portal will appear in the centre of the tower.