Now read this carefully: you can do what you want with the contents of that zip, provided you leave the license header alone that’s at the top of each file and respect its meaning (and the licenses of the various libraries). Make your own RTS or Tower Defence game, pinch any bits of code you like from within (in which case the license header is probably in a grey area but… I don’t care much), or maybe make some mods for Revenge of the Titans itself.
What you absolutely may not do is redistribute our assets – that’s the graphics and sounds and music – without our express permission, which if I don’t know you quite well, you are unlikely to get. Although some of the sound in the game comes from the incredibly awesome Freesound.org project, the oggs and jars are still ours; if you want the original .wav files which are licensed under the creative commons license, we’ve included links to every one of them in the docs.
You may also absolutely not use the name “Revenge of the Titans”, “Puppygames”, or “Shaven Puppy Ltd” when redistributing any derivative works without our express permission, as that’d again be stepping on our trademarks.