Fortify

Fortify is a real-time strategy game with a tower defense twist. Defend your castle from attacking monsters and siege weapons by using ranged and melee defenders, catapults, cannons, oil fields and more. You start at the iron age, advancing to the medieval age, and later, the imperial age. Each new age enables important upgrades and allows for new techniques to defend your castle.

The Steam Edition includes

Two mission maps Multiplayer Player vs Player, tower defense style Arcade mode: How long can you survive the attacks? Real-time lighting and shadows Achievements Leaderboards

And more

Multiplayer mode plays just like normal, but instead of having enemies attack automatically, every monster that attacks you has been sent by your opponent. In order to gain an advantage you must learn to balance attack and defense.