Tower Defense games are strategy games where you have to defend against waves of enemy attackers. They're coming, wave after wave, and you must build attack, defense or magic towers to welcome them as it should.
Tower Defense games will also ask you a perfect management of your finances because if you waste your money, you won't be able to build new towers.
Be aware that enemies always follow the same path so you cannot get an unpleasant surprises waiting for them.