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Beat the main game today with 55 feathers remaining and a smattering of medals (mostly bronze); found out what Ivy is.
The back 50, if I can go by the first few that I played, could be remixes of the first 50, with required keys to get to the exits and a whole lot more hazards placed everywhere. I would have happily paid $30 for it. (review copy, as I mentioned)
It'll take you maybe 1-2 hours to get through it first time, but if you're a score fiend you may want to try going for the whole quest again, as there's a high score table for runs through the main game. I took a little longer because after I ran out of lives (lives matter! wow!) in world 8, I decided to try again from the beginning to see how that felt rather than just continue. (By the way, don't be afraid that starting over threatens to erase all your datait's just talking about your continue point. Doesn't seem you lose medals, cleared levels, etc.)
This is a pretty neat game, all in all. It's got a reasonable dose of challenge, a very unique mechanic that takes a bit to master (tip: don't just make paths for Ivy to walk on; sweep her and other things around with the vinesalso, watch out that you
don't unintentionally sweep other things into her path!) and it's charming as well.
I'll play some more, try to get some medals, collect more feathers, work on the back 50, and post my review soon.