It adds a few new enemies and gimmicks that make the game more interesting than its predecessor and incorporates the 2.5D in its levels better instead of using it just for aesthetics. It's also much harder, as the later levels are long and have difficult jumps that can easily get you killed, forcing to do it all over again as there are no checkpoints. There was also a really difficult boss which had 3 phases and felt much harder than anything else.

Overall I liked Clockwork Knight 2 a bit more than the first one. OST is still amazing too.