If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Importantly, you must protect the gate and prevent zombies escape. In the game there are 6 types of weapons and 6 upgrades for each and 3 types of upgrades for tractor: life, armor and speed. You can find it in the shop. In the game there are 6 large levels with different number of gates that must be protected and a swarm of zombies. You can kill bosses and cannons and get good money for it. Beware of fat zombie, don't allow him to a tractor. To get a double points you must press the usual zombie by a tractor. Good Luck!
W,A,S,D to drive tractor. Mouse key to fire. 1-6 buttons to change weapon. R or Space bar to reload.