It's dark in the cemetery and there's a battle going on between ghouls. You're one of them! Accomplish all the missions at the bottom right: here you can see how many rings you'll have to gather, how many enemies you'll have to kill and how many turrets you'll have to place. Try to enter into 'Windfall mode'. This is a mode in which all you gather, counts twice! At the bottom of the screen you can see how many rings you've gathered, which day it is, what your score is and wether you're in 'Windfall' mode. Good luck!
Use WASD or the arrow keys to move around. If you press W or the up arrow twice, you jump twice as high. Use the mouse to aim and to shoot. You can hold the left mouse button for a continuous bullet stream. Press E to place a turret and press the spacebar to upgrade your weapon.