With approval from Epic Games, we have released a beta version of the Takedown Level Editor. Content includes all the environment assets, Killhouse maps, and HQ map (as reference to make your own TD maps, and hook up the game types/modes).
Due to time constraints, no official documentation is available yet. We'll do our best to answer questions and build documentation organically with the community. Anyone well versed in UDK/Unreal should be able find their way around just fine.
Don't see it listed? You must own a Steam copy of Takedown: Red Sabre to utilize this beta editor. This is for the purpose of using the editor in conjunction with the shipping build of Takedown, and to test your custom maps properly in the game.