Swarms are large hordes of zombies that spawn on the edges of the survival mode map. They always move to attack the colony's Command Center and are comprised of large tight groups of runner, or runner + special zombies depending on the length of the game.
Swarm Types
There are 2 types of swarms players will encounter during the game - scripted and random. Every time a scripted swarm is about to appear a window with a message "Zombie swarm detected near the colony from the x" (where x is one of the 4 cardinal directions) will show along with a timer that counts game hours left before it spawns and the music will change. Once it shows up it will be marked on the minimap with a yellow skull.
There's a total of 10 scripted swarms (including the final one) in every survival mission and depending on the settings they will appear sooner, or later and will have more, or fewer zombies. Waves appear on a regular schedule, and except for the final wave, are announced 8 hours before being placed on the map. The final wave is announced 24 hours before being placed on the map. The exact time that a swarm arrives can vary by up to 24 hours. The following lists the normal schedules for each duration.
Swarm Schedule
| Wave 1 | Wave 2 | Wave 3 | Wave 4 |
Wave 5 |
Wave 6 |
Wave 7 | Wave 8 | Wave 9 | Wave 10 | |
|---|---|---|---|---|---|---|---|---|---|---|
| 80 Days | Day 11 | Day 19 | Day 26 | Day 35 | Day 40 | Day 46 | Day 52 | Day
58 |
Day 65 | Day
73 |
| 100 Days | Day 13 | Day 23 | Day 33 | Day 43 | Day 50 | Day 57 | Day 64 | Day
71 |
Day 78 | Day
91 |
| 120 Days | Day 16 | Day 27 | Day 40 | Day 51 | Day 61 | Day 69 | Day 78 | Day
85 |
Day 95 | Day 110 |
| 150 Days | Day 20 | Day 35 | Day 50 | Day 65 | Day 75 | Day 85 | Day 96 | Day 108 | Day 117 | Day 138 |
Random Swarms
Random Swarms will start appearing after 2nd, or 3rd scripted swarm attack. These hordes are usually much smaller than those the game warns players about, but the game won't warn them when they spawn meaning that they might catch the colony off guard. The best way of dealing with them is to make sure that no side of the colony is unprotected, so that the damage they deal won't be too severe and no structures get infected.
Final Swarm
***SPOILER ALERT***
Once players are close to their time goal during survival games the message will pop up saying "Infected come from all directions! THEY ARE BILLIONS!!" warning them about the final attack of the game. This is the biggest and most difficult attack to repel of all swarms by far.
The final swarm is comprised of thousands of zombies spawning from all edges of the map including multiple Harpies, Venoms and Chubbies as well as all other zombies on the map that weren't killed thus far meaning that unless the colony has incredibly strong defenses prepared by this point it most likely won't hold out. It also makes clearing of the map of all remaining zombies and Villages of Doom very important since it will mean that less enemies will come in to attack.
Tactics
Like all swarms, the Final Swarm is heading straight for the Command Center. So it is best to build a good defense around it with at least 2 to 3 layers of stone walls, spikes and wire fences. Be careful about leaving too much space between your North, East, West and South defense lines, because you may want to swap your unit distribution among them.
Every building which gets destroyed by the zombies will spawn several more, equivalent to the number of workers inside it. So you should sell buildings in the way of the zombies (which is nearly every building not inside your defense lines). This also gives you a ton of resources to bolster your last defense. To avoid unit desertion due to the lack of gold, you can build up 2 to 3 warehouses inside your last defense or way in the corner of the map, to stock up on gold.