Nailed the NES feel for this in every regard!
Rat mode is soooo fun! And the first boss was mad challenging. It felt SO great to get that upgrade for defeating the boss. The transformation mechanic was really unique and fun! Turtle mode was cool too but felt a bit too slow to actually use. Maybe if it made me impervious to projectiles... that could be cool! I also tried to go turtle mode to get past the bear traps, but that didn't work sadly.
I was able to defeat 3 bosses before I got stuck. Really wish I could have found every boss, but I still had a great time playing! The only way I could see myself getting further was grinding 100 coins to buy one extra heart to get the key with 1 heart left... and i really didn't want to grind coins that long. Sadly I lost most of them after the boss kills and I was close to 100!
Here's my gameplay experience, getting lost and wandering around for 8 minutes :P