Tower defence
Also said as TD
Enemies walk from one side to the other, and you stop them by placing towers that fire on their own. You never touch the enemies. It promises the joy of watching a plan you made work.
It moves all the skill into the placement. Every tower is a decision about where to spend, and the waves are a test of it, which makes it easy to learn and very hard to put down.
Say it
“Enemies path around towers, so allow mazing, but refuse any placement that seals the route. Start with two tower types: fast single-target, and a slow that hits an area.”
Where you’ve seen it
Desktop Tower Defense (2007), a Flash game by Paul Preece, had enemies path around whatever you built, so you made them walk long mazes. Plants vs. Zombies (2009) moved the idea onto fixed lanes, and took it to a far wider audience.
Not to be confused with rts
An RTS has you commanding an army and attacking. In tower defence you never command anyone: you only build, and it plays out.