Recently got Castle Panic and have thoroughly been enjoying both the playing as well as thinking up variants, and here is my favorite we've thought of so far:
Use a timer to limit each players turn. This will add a fun real time element to the game, greatly increasing the frantic-ness of it all! Start the timer before you draw cards, and then after the timer runs out, you are forced to move to the next phase, moving the monsters in and then drawing new ones.
We haven't nailed down a time length yet, but thought that 1 minute would be a good place to start. What do you think?
This variant sounds really fun, and it is only a small change to the actual game! Have you tried it yet? I would imagine using a single timer for someones entire turn after drawing cards could be more fun, so steps 2 - 4 of the turn are timed all together. Then monsters move and the next person draws up and the timer flips again! I may have to try this variant next time we play co-op instead of with master slayer rules...