What is the core gameplay loop you envision here? Cause I can come up with a few ideas, but if you have specific ideas they probably won’t apply. Also there’s a particular balance to be struck: if you hamstring the player too much it won’t feel like a game, and if you give them too much leeway they won’t feel trapped.

The first idea that comes to mind is: the princess is a sorceress who is trapped by magic to the tower which means she can’t leave and doesn’t have access to her full magical powers (you could include a progression mechanic where the more bosses the knight defeats, or the more magical crystals he shatters, or whatever, the more you can help him.) But she’s still scrying on him, watching his progress, throwing the occasional beneficial spell or nuking crowds of dangerous enemies before he gets overwhelmed, etc. The reduced interactivity will make the player feel trapped (and slowly less so as the game progresses). But maybe the scrying window is smallish so you can’t see the whole field at once, and it slowly grows as her power increases.