I posted these screenshots in a Brazilian SMW hacking Discord server, and one of the users suggested me to put custom palettes and layer 3. I didn't do this because I'll only put custom palettes when needed: such as the player and the green switch in world 1 (it replaced the original Yoshi's Island map).
After a week-long break for no reason, #3 Lemmy's Castle is finally done:
In this level you use the net doors to get past the red ? block barriers.
Also, someone told me that the lava looks bad due to the brown palette, but in the original game there's lava with these colors, such as in the level Chocolate Fortress.
And before the second demo is released, I must make the yellow switch palace and the world 4 map first.
All the levels in this world will be underwater, except for the secret level.
Also ignore the yellow path to the right of the level with a pool, I've removed it. I've left only the white dots in the water and the path to the left of the castle.
Now I'm gonna begin testing the hack before submitting it.
I ended up adding a custom sprite for this level: a chasing Porcupuffer, as the original sprite would endlessy swim up and go offscreen. I also modified its speed so that its not very hard to dodge it.