If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Its Zombies madness again... City is under zombie attack, zombies everywhere get into the action and kill some zombies and get to the roof top which is at 20th floor get upgrades buy weapons and special power. Bosses after every 4 levels. There is also blood on/ff and you can use red blood or green blood. Enjoy killing zombies.