Looking back on it there are 4 of each room (1-38) ... the tower is also 38 rooms high (6 visible on the ground map, 32 on the tall one - I think). It might be that each time you beat one you're warped one floor up the outside, and then you can go in as soon as one of the 4 stages is cleared :p
There are only 32 bosses though...
There are only 32 bosses though...