Lock & Key is a tower defense game. With only one attacker wave. And it's in text.
The premise is that you're a dungeon designer, the one who lays out the arrangement of traps to keep in the extra-specially-dangerous prisoners. You've got a grid of rooms in which you can place these traps, and a limited budget to spend. When you're done, you're taken aside to a guard room with the King to watch as Boldo -- a kind of Tarzan figure gleaming with oiled muscles -- does his best to break out. If he fails, you win. If not, you get to watch Boldo defeating all the traps you so carefully laid out -- and the consequences for you are disastrous. Time to play again.