Maybe I got some glitches or something, but...
1) Upgrading runes in the shop seems to do nothing. Damage doesn't seem to have increased, and they're nearly maxed out.
2) Two levels before the tower, I ran into an unknown number of dragons. Stopped counting somewhere in the thirties. At one point, they filled up the right side of the screen in the space of about ten seconds and steamrolled me with fireballs.
3) Goldrush mode also seems to do nothing.
I'd like to finish this game, but for whatever reason I seem to have hit a brick wall. It's been otherwise a fun and enjoyable time-killer.