It is going to be able to get five hundred gold the challenge that prevents you from spending such an amount that you can spend so much gold on a single tower. This requires you to save and plan the entire game over, likely having to forgo improvements on your defenses for several turns in a row. The best way to go about this is to put it off until the later levels, somewhere in the mid twenties. Go ahead and fortify your defenses, but do not blow your entire bank each time. Try just adding a couple every two or three turns, that way you end up with an accumulative surplus. Do not research the earthquake tower right away, save it for later on. The first two opportunities for research, meaning you have acquired two halos, should go to increase your interest rate. This will let you have the money later on when the time is right.