Congrats on entering your first game jam! This game is really impressive for a first entry.
The whole invisible level gimmick is very effective, as it is both easy to learn but also can be used to make some very difficult levels. I don't know if it was intentional, but having everything be grid-based made it much more fun to play as I could more accurately plan my route.
While most of the levels were designed quite well, I'd recommend having super easy levels before you introduce each mechanic (kind of like what you did with level 5). This allows the player to get used to new mechanics on easier levels, before you use the mechanics in more difficult ways.
Overall, this is a really great game, and you should be very proud! It kind of reminds me of old flash games, which is cool. Great job :D