Has anyone tried Tower Wars?
Tower Wars has two modes. There's the normal mode where you have to split your resources. On one hand you have to build and upgrade the towers defending the paths to your castle. On the other hand you have to build and improve the units you send to attack the enemy castle. Meanwhile the enemy is doing the same thing.
You can play against the computer, it's not just multiplayer.
There's also "classic" mode where you play pure defense only while the computer sends waves of enemies at you that get stronger, faster, etc. There are 3 maps for this and it keeps track of how long you stay alive as your high score.
I personally didn't like the first mode, where I had to attack and defend at the same time. But that might be the thing that would keep some other player from getting bored. If you can get a copy cheap (I think it was bundled before) you might give it a try.
I did enjoy the classic mode, and played it while waiting for the Steam trading cards to drop. You eventually reach a point where there's no more room on the map for towers, there's no more upgrades you can buy for your castle, and you're just waiting for the enemy waves to get strong enough to smash your defenses. There's also only 3 maps. There are multiple paths and some enemies will leave the path and find alternate routes if you let them, but still... only 3 maps. I wouldn't buy the game just for classic mode as other TD games give you so much more of this style of game play for your buck.