Maybe add in "premium items" (still use gold, but price based on your level) could include things like double drop rate, double exp rate, temporary stat boosters etc..
I'm confused as to the start of the game how I hit Brown Town Rats and Foxes for 1 damage, yet the Town rats can usually critical hit me for 800+ damage?
 "Battle without End"){kind=icon}
 "Dawn of the Dragons"){kind=icon}