Castle Games

A castle is the perfect defence-game object because it explains itself. There's a thing worth protecting, walls to protect it with, and something outside that would rather you didn't.

Castle Defender Saga is the most complete of them, with progression behind the waves so a session builds rather than resets. Warlock King - Castle Defense adds magic to the defending, which changes what your best answer to a wave looks like. Tower Defense is the format at its plainest, and it's a perfectly good version of it. All three reward planning between waves far more than they reward reacting during one, which is the format working properly.

There are 3 free games under this tag, all playable in the browser with nothing to install, and all three reward planning between waves far more than reacting during them.

The full format is Tower Defense games, and Tactical Games covers everything where placement decides it.