You are being besieged by zombies. You can set up blockades to hold them off for a while, but you will have to shoot them all. Before you go on to the next level, you can buy upgrades like ammunition and weapons, or even mercenaries. Click on the mercenaries to order them around, they can stay or follow you. Good luck!
Use WASD to move, Space to jump and the mouse to aim and fire. Press I to open your inventory, E to fix a damaged barricade and 1-4 to change weapons.