In my book i would make the following rough distinctions:
- a path leads to your `center`
- enemies come on that aroundabout path to your center.
- you place defense units along this path
- the center itself is usually undefended and cannot be rebuild or upgraded.
- you win when the enemy waves are defeated.
- you lose after a certain number of enemies hit your center.
- you (try to) kill the enemies as they approach your castle in direct ways
- with your only defense IS the castle.
- you can level up your castle and itīs units etc.
- when the enemy reaches your castle,they start to damage it
but you still can kill them off.
- you loose when the castleīs is destroyed.
- you win by defeating the enemy waves.
- there are two opposite bases on the playfield.
- throwing units at each other
- which fight when they meet somewhere on the playfield
- the winning unit moves on to the enemy base.
- units level up, you base usually not.
- the one destroying the enemies base first, wins.
example: Alien9, Armageddon
update: what they have in common is that they are usually in real time, not turn based.
now these are the basic concepts only*. Some other games
just bend the concepts as they see fit or add RTS and other management features.