Medieval build-'em-up Besiege offers a lot of physics-based shenanigans for not very much money, letting you build vaguely Robot Wars-esque siege engines, before letting them loose on Medieval villages and fortresses. Released at the start of January 2015, it's still in early access—however, there's a big update coming that should breathe new life into the game.
The update's named, rather grandly, the Besiege Multiverse, and it will add co-operative and competitive multiplayer, a level editor, and an open sandbox you can mess around in to your heart's delight. Here's a trailer:
Excitingly, you'll be able to build levels while your friends are in there playing them. You'll also be able to fine-tune things like the physics and stats of units placed in the world.
There's no date yet for the Multiverse update, but developer Spiderling Studios hopes to have it released to the public by the third quarter of this year. It'll be free if you already own Besiege, while the price of the game will increase by $2 to accommodate the new features, when Besiege Multiverse arrives.