Great game. I really enjoyed that the castle didn't have health but instead the level just looped repeatedly until you reached your killstreak.
After beating the game normally, I found myself wishing for a "race" mode. In this mode, I would be able to prestige and collect the bonuses in any order (instead of having later ones locked by pre-reqs), so I could try and beat the game in a certain number of loops without all of the bonuses. To that end, it'd be nice if the victory screen showed some stats like time, total number of gems collected, number of times each level was completed and the like. Then I could screeenshot my accomplishments and post them here in the comments.
Even without that, I found it fun to replay the game after beating it and trying to beat all the levels by only focusing on a specific tower type.