I spent some time recently playing Besiege, an Early Access game in which you blow up castles, massacre tiny soldiers, smoosh sheep, and accomplish other objectives with siege engines you build yourself. Snap together parts and pieces, add wheels, wings, and weapons, and wreak havoc in an enjoyable and attractive construction-slash-destruction sandbox. Screenshots didn't feel like they would do the fun physics justice, so have a look at the video to see the game in action.
Besiege is in Early Access and can be purchased DRM free from Humble or DRM-laden from Steam. Here's the official site's store page.