They came in many flavours, but Cube Defense is fairly representative.
It's a bit like Tetris, in that the game proceeds in levels. Each level, a wave of enemies would run from their spawn point to the centre of the map. All you can do is build defense towers to kill them off as they run through your build area.
Most waves are ground units, and you can arrange your towers in such a way that those units must spend more time walking through your area, which means each of your towers can fire more times. Every so often there would be an air unit round, and this maze approach doesn't work. Some towers have special attributes, like slowing an enemy unit with cold damage.
It's a bit like Tetris, in that the game proceeds in levels. Each level, a wave of enemies would run from their spawn point to the centre of the map. All you can do is build defense towers to kill them off as they run through your build area.
Most waves are ground units, and you can arrange your towers in such a way that those units must spend more time walking through your area, which means each of your towers can fire more times. Every so often there would be an air unit round, and this maze approach doesn't work. Some towers have special attributes, like slowing an enemy unit with cold damage.
https://www.youtube.com/watch?v=FeJ7IrE7aFA