If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
When the farm is under attack, it's time to put your hired help to REAL work...as towers of defense.
WASD keys to scroll map.
Click to select and place towers, keeping the enemies at bay without letting your health drop to 0. If you're ready for more enemies before your prep time is up, click for the next wave to earn some extra money.