Level selector is a good idea actually! I'll add that and level indicator when jam ends and itch will let me update game files.😉
So the game was all made in Construct 2, but I edited some sprites in MS Paint. 😅
For the level design, in the beginning I just added simple square boxes. It looked kinda boring, so I made a tileset with those boxes in different shapes and colors and added them randomly. I just made labyrinths out of them so the first challenge for a player will be just getting from start to finish. Then I added switches, doors, and other puzzle elements so the player gets a harder challenge. In the end I added enemies and water so the player gets danger added to the challenge. As a bonus I added collectables, even though they are optional, player can still risk trying to get them all. That's kinda everything. I tried making levels more complex as the player got more experience in the game so first levels look empty and last ones look like nightmares, but by that time player is experienced enough to complete them. Biggest tip I can get is that every level needs to be tested a lot. Hope that helped a little!😉