Gravity?

"Huh, zombies." Johann raised an eyebrow, unsure of what to make of the situation.

The rest of the participants narrowed their eyes while the loud voice of the announcer reverberated across the dimly lit streets.

"Run through." Johann kicked the horse's flank and made him neigh in pain, but he increased the speed and ran over one of the zombies.

Once the horse's hoof stomped on the zombie's head, it exploded. 

Blood and guts splattered everywhere.

"Heh, marvelous." Johann grinned and was the first one to enter the narrow alleyway, where only two riders could fit side-by-side.

"Gruh!" A rotting zombie screamed and leaped like a jumping frog at the monstrous-looking Nathan.

The demonspawn snarled and punched through the zombie's decaying head with a sickening squelch.

The destroyed head painted the nearby walls and windows in green blood and brain.