I'll try to ask this without spoiling anything huge.
After the final boss fight, in the final room there's a magic circle, with one of its unlock requirements being to beat a certain boss.
How do you fight her? Is she a random encounter in the tower or is there a secret involved?
While i'm at it, where's the key that unlocks the cell in the laboratory?
The magic circle, I believe, refers to Theresa, the blonde priest-like woman you can bump into in the tower (the one with a holy book in her room in the inn's first floor). You can bump into almost all the female NPCs in the tower, your interactions with them are, I believe, random. Sometimes they'll chat, sometimes give you something, sometimes be overcome by the tower's aura and want to fight you. So really you just need to run the tower until you bump into her and also get an option to fight her. She asks if you want to be 'healed', and you get a yes/no option, yes starting the fight. She's pretty easy to beat, one of the easiest friendly NPC battles. You also get a really great skill for beating her.
That magic circle event also requires you to have donated a lot of money to her, for healing services, which is another random encounter with her during tower runs.
The key that unlocks the cell in the laboratory I believe you get from the cat. If you bump into the cat during runs in the tower, it'll show up outside the lab at night. Talk to it at night and it'll give you the key to the cell.
Some of this could be wrong, going by memory, but should be correct.