It's nice, but exponentials screwed it up. You sell a golden ingot for 2000 gold, and 2500 for 1e+23 gold. LOL? I have 1,8e+18 gold (1,8x10^18) and can't damn buy anything. When I bought the items I have, I was fighting monsters with over 1e+20 HP, which killed me in a single hit lots and lots of times, and now I'm fighting mobs with 90k HP :P
That's how an actual donation system should work. You CAN get EVERYTHING, without paying real cash, including every single upgrade, for every single unit.
I noticed that the Flying Castle is pretty glitched. Sometimes, when you kill it, the game freezes, only the hero and the castle can move, and they are both invincible. Also, is the castle intended to be in the game indefinitely, without heading to the end?
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Well, the only point is to scare players and to catch them off guard.