If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Defend your city against the underground invasion by strange creatures which are the resulting of toxic spills from a chemical industry. The creature are in sewage and the bar is the only thing that separate the sewer from the city. Destroy these creatures while defending the bars from being destroyed by the creatures. Kill as many creatures and earned money which you can spend on buying upgrades for improving your defenses, turrets and weapons.