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. Here are a few ideas off the top of my head:

Direct but limited intervention: the princess is a sorceress who is trapped by magic in 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), and maybe the scrying window starts out smallish so you can't see the whole field at once and it slowly grows as her power increases.

Distraction/misdirection: The knight has made it to the tower/castle and has to fight his way through the guards, but instead of attacking the guards directly you're trying to cause a ruckus to distract the guards so he doesn't have to fight them all at once. This could even be a stealth game where you're knocking things over and banging pots and pans or whatever to distract the guards as he sneaks by them.

Puzzle game: The castle/tower itself is magical and has floor tiles/walls that can be moved around, and the princess is manipulating the castle around the knight to give him the best path through obstacles and to limit the number of guards that can get to him in any given room.

It really depends on what kind of game you want to make here.