Survive the Endless Horde!

Take this as an actual campaign:

A endless horde of undead is attacking your city from all sides.

The horde will get upgraded over time, but so will the city. Your goal: defend.

Each and every user can work together. You may use any spells you like so long as the current spell level permits it (including homebrew spells).

Current spell level: 1.

Magic Missile everything until I run out of spell slots, then I spam Poison Spray every turn. All from the (relative) safety of the walls. 

I guard the north gate, my blade at the ready and my spells prepared. I cast Wirebolt (like the one from the Power Word Kilt forum game) upon a zombie, landing a direct hit and dealing 5 lightning damage.