The bigger your horde gets, the harder the town fights back. Police cars, SWAT vans and army trucks now roll in to stop you, and new humans no longer walk straight into the horde.
Armed response - the bigger your horde, the harder the town fights back: police past 60 zombies, SWAT past 150, the army past 250
Police car, SWAT van and army truck - drive in along a road, park where there's room and drop off their squad
SWAT - riot police in body armour, with their own lines
Squad bounty - turned squad members are worth triple brains
Sirens, a bugle call and car doors - you hear a squad before you see it
Spawning - new humans come out of the door with the fewest zombies outside, so a big horde can't camp the doors
Late game - squads scale with your horde's bite and health, so they stay a threat however far you've upgraded