Level 2 wasn't as hard as much as it took a bit to adjust to, also the camera is too close so enemies can sneak up on you.
Level 3 was really busy, I think segmenting it into different themed areas could help with telling if a key is in a radius or not, also I hadn't noticed the door until I got three keys (gamers never look up)
The ice spawned too much especially with the boss, I found fire and rock were the least likely to spawn for some reason.
I never figured out what was the start button.