The Attack
The Attack of the Hordes takes place every night at midnight (23:00 game time in the UK version), the zombies attack the town and attempt to kill citizens. Citizens who die outside during the night will be added to the horde. The citizens in town must build various defences to keep the shuffling, groaning masses at bay.
How many zombies get in is simple. If the town's defense is more than the amount of zombies attacking, then you will be safe and the citizens inside town will suffer no damage. If the town's defense is lower than the amount of zombies attacking, then some zombies will be able to enter town and kill or terrorise the citizens. Note that if the gates are left open, then the town defense does not matter and all the zombies will make it in.
The attack has a relatively predictable pattern. It will steadily increase for two to three days, then suddenly "spike" approximately every four days to hope-crushing heights. On this spike, the size of the attacking horde will be larger than average, and it may be difficult to gather the needed defense. The day after the spike, however, the attack will return to average or below average for a day, possibly even decreasing in number temporarily.
Any zombies who make it into the town are randomly divided between the members of the town. Resisting these zombies is the duty of the defense rating of your house; 1 point of home defense = 1 zombie turned away. If you are assaulted in the night, but survive, you will start the next day 
Terrorised.
However there is a small chance that you were attacked but did not get terrorized. In which case, the message from the Crow will be as followed:
"You were really sweating last night! You heard them groaning, banging around and yelling outside. Eventually your door gave way and you took refuge under your bed, clutching your remaining possessions, trying not to think about the situation. Then they left... Shaking with fear you eventually came out of your hiding place, grateful that you survived."
Lazy zombies
Starting from Season 3, zombies may not attack anyone should they manage to get into the town. Here is the formula for if they will attack or not: Day 5 or below: 70% of the population is "protected“ and is not attacked by zombies in case of overflow (this limits carnage in small towns, though it doesn't prevent all deaths). Obviously with gates open many more zombies get in, so you still must close them to get less casualties.
Day 5 and on, open gates: No immunity (protection), only personal defenses count (less moral, more violence)
Day 5 and on, closed gates, more than 20 survivors: About 40% of the population will not suffer under an attack in case of overflow. This allows an advanced town to survive an attack spike at the cost of a few human lives (you can limit the loss by upgrading your homes).
Day 5 and on, closed gates, less than 20 survivors: No immunity, all zombies attack a citizen.
Keep in mind that in case of overflow the number of zombies attacking citizens is always limited to 30*number of residents alive (f.e. 35 alive, *35 = 1050). Those zombies are then distributed randomly (some houses will be attacked by 4 zombies, others by 50).
Special messages
If, during an attack, zombies get in (it doesn't matter how many) and no one dies, it will be called "a miracle". If, during an attack, zombies get in and only one citizen die, it will be called "a stroke of luck"
What can I do during the attack?
During the assault, the game is inaccessible for 10-20 minutes, during which time a video will display instead, with two randomly chosen actions. Clicking either action refreshes the page and brings up other actions. There's no real value to this; just something to amuse yourself while waiting to see how your town fared.
List of actions:
- hug your rucksack
- scream
- wail like a banshee
- grind your teeth
- hug a broken guitar
- cry nervously
- scream and panic
- hide behind the wreckage
- hold the door
- wait under the sheets
- call for help
- yell “help”
- hide in a box
- hide under the bed
- wait calmly...
- curl up in the fetal position
- sing at the top of your voice, alone
- pray for your life
- clench your cheeks