The goal of this game is to defend your base against monsters by building defence towers. Monsters will steal your gold if they get to your base. If you don't have enough gold when they reach your base then the game is over and you have lost. If you succesfully resist all monsters then you have won.
Towers are build on the grid. The grid consists of tiles and some tiles can give specific bonuses to towers. Tiles that give one or more bonuses to towers are colored. Squared colored tiles give positive bonuses (e.g. descreases build/upgrade cost of a tower). Circular colored tiles give a negative 'bonus' (e.g. increases build/upgrade cost of a tower).