Tap on the mark that has an (!) mark to start a level. The ones with (?) you can't enter, unless you complete the other levels before the one you want to go to. Each tower costs 200 gold coins and you can also upgrade them to make them stronger. Remember not only do you get stronger so do the enemies so be careful to watch out for them! Good Luck!
Thankyou to @ShiftClickLearn for the original project!
