great!
The game was able to keep me entertained through its entirety and I thought despite the relatively simple graphics, the story, gameplay and music were good enough for it not to matter. One glitch I found was in the castle level in the floating bookshelf room, where if you feel down the first immediate hole, every time you respawn you'd be invisible and automatically die again. I'm not sure if this is a player issue as you say before, as I was able to play through the first time without a single problem and this has only just occured after trying to get the secrets.
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To find secrets on the boss levels; during the cutscene after their death, there's a time when lots of floating question marks appear at one side of the screen, clicking this area will get you the secret ^_^ Its just after you take the key from them.
If anyone knows where the 4 secrets on the castle level are by the way say so because i'm lost :|