If you have played Bastion, then the same developers made Hades. The game is a rogue-like where you control the son of Hades, Lord of the Underworld. Your mission is to leave the underworld by going through many arena-like stages. If you die, you restart at the beginning, but you get to keep your skills, upgrades, and weapon unlocks. The game is truly fantastic.
In Besiege, your main task is to create your own siege weapon. That's literally it. You have building tools and you use them to create your own unique siege engine. In each level, you will be given a target to destroy. Your siege engine should successfully destroy that target. You can make a catapult, a flying machine, a drill machine on wheels. It's your choice.