I love this concept, particularly if 4 civilisations are allowed to build up from the ancient area in each corner of the map before a central zombie hive outbreak is released in the middle after someone reaches the future era. This gives incentive to a science race while building up defences, before the last player standing wins.
That way, players can choose the most defensible positions on the map and war with each other prior to the zombie apocalypse to try and weaken each other. In some ways though, it would be best to protect the players in the corners nearest you and hope that the player on the other side of the map is overrun first - resulting in you being in the strongest position. So it will be quite political and strategic.
I certainly agree that zombies should convert player's units into zombies and that overruning a city spawns as many units as the population. Also, pillaging farms, mines, etc should strengthen them as previously described, as they've converted the farmers, miners, etc. We need to remember that when someone starts to be overrun, things fall apart very very quickly unless strong reinforcements arrive. It also makes city placement very tactial; because if you position a small city poorly, it will easily be overrun and spawn numerous new zombies close to your borders.
I like the idea of a zombie culture tree, giving them small bonuses to improve their strength. I also like the idea of using religion as a form of the virus, giving it multi-level impact with inquisitors and missionaries.
Effectively, this should be an unsurvivable scenario, with zombies multiplying at exponential rates. Strong defences will slow them down, but will never be able to stop them due to converting ex-troops, cities, pilaging tile improvements and an increasing rate of spawn from the central hive. Each zombie should be programmed to directly attack a certain civilisation, with equal numbers of zombies heading to each. Therefore it won't be down to luck (except starting location), it's more down to planning and politics. If zombies overrun your city though, every new spawning zombie from that city will only attack your cities, not head to other players.
Also, just imagine the possibility of launching a strong flank attack on a neighbour just as the zombie apocalypse hits!
Great idea!
