I’m not sure who came up with trapdoors, but they’re… well, I’d call them mildly annoying. It takes three hits to destroy a trapdoor — you get rid of them the same way you get rid of balls or beetles — but while they sit there, balls and dots can fall through them. This can make it a bit of a pain when you’re trying to build a square, because if you’re not careful, you’ll clear a trapdoor and your square gets ruined.
Trapdoors block water (and cannot become waterlogged, so to speak), so if you’re on a water level, you need to keep an eye on where the trapdoors are and what they’re keeping from moving around.
As I said before, watch out for having to clear backgrounds and spread water, and also watch out for slime and fire, because trapdoors will restrain them. Beyond that, it depends upon the level’s specific goal.
If this guide helped you out, please consider buying one of my stories from Amazon. Here’s one: My Pillow, a horror story (so no one under 18 should read it) originally published by Abstract Jam. It’s basically about a guy who gets a body pillow, and what happens when people transfer bad emotions to inanimate objects.