Yes. Use any Java bytecode decompiler, preferably one that can read .jar files directly.
In Java ecosystem most of the optimizations are done on the JVM side, so source code to byte code translation is almost 1:1. So, unless source code was obfuscated, it can be easily restored from bytecode. Of course some bits will be lost, like comments, source-level annotations and such.
I've used something called jd-gui-1.6.6.jar, I don't remember where I got it.
BTW, the code sucks. But it doesn't matter, because it gets the job done, and this game, with its general idea, writing and mechanics is awesome. So I still have huge respect to CSdev. I really wish that most of this ugly bytecode is auto-generated but I know better.