Enchanted forest, dark and swampy like, zombies come out of it during the night, villagers all hide in houses, players job is to defend the village, whether with bow and arrow or melee, or cannons if someone can afford to purchase an upgrade. Have there be like a server store where people can buy upgrades, like a small wall, bigger wall, parapets, health potions, cannons, lookout towers, ditches, etc.
or the other posts, those are nice too.