Manage food supplies, housing and morale while defending against undead attacks. Reclaim the city one square at a time and put your survivors to work scavenging for food, building houses, rediscovering technology and of course killing zombies.
You’ve got to purchase turrets and drop them on the orbit, aiming at the advancing space bodies. If you destroy an asteroid or comet then you earn some money to buy more powerful defensive weapons. If Earth is destroyed then your game is over.