One of the best PS1 games I ever played.
You play as Millennia, a girl who was kidnapped at young age and raised by an immortal (Not really immortal, more like ageless) alien-like race called The Timenoids. They taught you how to use traps to defend yourself (and attack foes) and use you as a bodyguard to do their dirty work: Killing the adventurers who come to seek for the TMD's immortality secret.
So the game puts you into different places you gotta protect while swarming you with different kinds of enemies who will try to get to the TMD secret. Some of the enemies will attack you, some others are just curious villagers. They'll get more dangerous the more levels you pass.
To protect the secrets you got a lot of traps to use divided into three categories: Floor traps, ceiling traps, and wall traps.
Every enemy you kill, specially if you do it with "combos" gets you "Arks" which is the game's currency so you can get new traps.
To get combos it's pretty simple: To give you an example... You put a magnet in the wall which will attract the enemy, then the moment they touch the wall you activate a beat trap in the floor that will give you some time while they try to get out so you activate a ceiling trap, an Iron Ball, that will go down a stair to finally getting attracted by the magnet and hit the enemy that was also trapped in the bear trap.
The game let you use your imagination and tools of destruction as you please, and rewards you for creativity.
You can't defend in any other way, just using the traps. Millennia can't punch, got no weapon, anything. The moment you get close combat with your enemies if you got no traps set near you you'll have to run to another room where you already setted up traps beforehand. (You can move and put more traps but there's a limit of active traps per room, and activation takes some time you'll be running away from your enemies).
A pretty unique game.