Here you can download new player-created levels, and upload your own.
You'll need to put new levels in the right place for the game to find them. Click on Open levels folder in the game to open the levels folder. New levels need to go within a subfolder of this folder. You can create new folders, named (and nested) as you like, to organise your levels. Zip files need to be extracted for the game to be able to use them.
No single-player build levels have been uploaded yet.