In your fight against the Zed you are asssisted by different teams of Nuevos Aires mercenaries.
By default the player starts a campaign with a single unit, the Evac Helicopter. New units are obtained by successfully completing missions. New types become available in random order. By default recruitment ceases once you've got one unit of each type.
Only up to 4 units (5 later on in the game) can be taken into combat. If you have more than that available they are chosen randomly at the beginning of the month, with one spot reserved for the Evac Helicopter. Note that this means only 3 (4) units are available while dealing with level-4 outbreaks, as the Helicopter cannot be replaced.
All of the above can be altered in various ways at the time of starting a new campaign by setting appropriate modifiers. Units gain experience in combat. Every time enough experience has been gained for a unit to advance a level, it is possible to improve one of its abilities. The game refers to this as upgrading.
Many units can be destroyed in combat. There is no penalty for that per se, however this means the player must wait for another unit of the same type to be recruited, then train it from scratch.
Machine gun-toting foot soldiers, protected from being infected by the Zed by attack dogs. Fire continuously at the closest Zed in range, managing one shot every 0.3 seconds. They do not harm civilians. They have to be placed somewhere on the map but can be ordered to move during combat. They keep on firing while walking. Infantry can be destroyed by explosions. Gain experience by: killing Zed. Upgrade capabilities: accuracy, movement speed.
At the beginning of a mission they are placed on a rooftop somewhere in the city and remain there throughout. From their building, they aim their shots along a line from the building in the direction of the aim point. The target area is larger if they are on a large building, and prefer attacking Zed closest to the Aim point. They shoot at a constant, slow rate, killing one Zed with one shot. They do not harm civilians. The aim point can be repositioned at will during the mission, however this takes time (constant regardless of how far the point is moved from its former location) during which the snipers do not fire. Destroyed when the building on which they reside collapses. Gain experience by: killing Zed. Upgrade capabilities: refire speed, repositioning time.
The main workhorse of your arsenal. At the beginning of a mission, the player places the artillery crew on a rooftop somewhere in the city. During combat, it can fire at any location on the map except for a circular area surrounding its location whose purpose is to protect it from its own shells. After its target has been specified the crew spends several seconds preparing the strike (which means one has to aim *ahead* of moving Zed), then lobs an explosive charge at the specified location. A single explosion has got the range roughly equivalent to the size of the targeting circle. Explosions can destroy buildings, however it usually takes more than one shell to achieve this. Once all available shots have been assigned targets, artillery takes some time to reload. Destroyed when the building on which the crew resides collapses. Gain experience by: killing Zed. Upgrade capabilities: targeting delay, reload time.
In addition to the regular upgrades, the artillery unit can fire one more shell between reloads when it gains a level. The number of shells per strike can also be increased through research.
Remotely-triggered explosive charges. Placed in the streets at the beginning of a mission, exploded when triggered by the player. Charges explode one after another, with short delay, in the order they were placed. Rather short range. Can easily destroy buildings. Gains experience by: killing Zed. Upgrade capabilities: number of charges.
The mercenary can deploy four explosive units, and can be upgraded to handle 10.
Placed in the streets at the beginning of a mission, explode when stepped on by Zed. Range similar to single-shot artillery strikes, somewhat stronger. As a result, very effective at destroying buildings. Gains experience by: killing Zed. Upgrade capabilities: number of mines.
The number of landmines can be upgraded to 8.
- Evac Helicopter.
Evacuates civilians and scientists from and guides them through city streets. Flies from outside the map to the specified touch-down point, sounds its foghorn (which causes all civilians and scientists within its range to start running towards it) and lands to collect people reaching it. Takes off after X seconds, once its full capacity has been reached, if any Zed reach the landing zone or if told to do so by the player. Once back in the air it flies out of the map and the cycle begins again. The touch-down point can be relocated at any time, however doing so while the helicopter is already on approach will increase the time before it actually lands (the more the closer to its original destination it was). Gains experience by: having civilians rescued (also by having all Zed killed - when that happens it is awarded points for all surviving civilians, not just those it picked up). Upgrade capabilities: approach/departure time, loading time per person. Note: helicopter capacity can be increased through research.
- Zed Bait.
Once placed in a street it attracts all Zed within its area of effect to its location for some time, then goes off. Takes time to reload before it can be used again. Gains experience by: attracting Zed. Upgrade capabilities: range of effect, duration of effect, reloading time.
Divert foot traffic, human and Zed alike. Placed in the streets at the beginning of a mission; they mustn't be placed in a way which would completely block some way into and out of the city, even if there are no Zed coming that way at first. Indestructible - but keep in mind that they do nothing to improve structural stability of surrounding buildings. Gain experience by: other units gaining experience. Upgrade capabilities: number of barricades.
The mercenary team can deploy two barricades, and can be upgraded to 5.
Three special weapons exist which are governed by different rules. While rather different from each other, they have got two things in common: they are unlocked by reaching milestones on the Victory Track, and they do not gain experience.
- Catbird Cannon.
An orbital launcher whose shots raze large sections of the city - along with everything and everyone in it - to the ground. Available in every mission after it has been unlocked (does not consume a mercenary slot), however each shot must be "unlocked" by evacuating civilians or killing Zed. The unlock rate can be improved through research.
- Elephantbird Cannon.
A vastly more powerful variant of Artillery: even at the very beginning it comes with a three-shot clip, fires X shells per strike, reloads very quickly and has got very short target-fire delay. As if that weren't enough, both the number of shells per salvo and the clip size can be increased (simultaneously) through research, with the most powerful level featuring truly monstrous 24-shot clip and X shell per strike. Only available in months for which the game has drawn the "Test Elephantbird Cannon" property, in which case it replaces all units except the Evac Helicopter; this happens on average once per game year. Like regular artillery, it can be destroyed by collapsing the building on which the crew is located.
Despite occupying the only mercenary slot available, in can single-handedly stop any zombie attack on a territory, guaranteeing a captured territory.
- Llama Bombs.
Weaponised llamas (don't ask). Obliterate all Zed in target territories, returning them to their original state. Can clean multiple adjoining territories at once, if aimed correctly. Unlike all other weapons, llama bombs are used on the world map. The player is given a payload of bombs (2 by default, number can be increased though research) at the beginning of months for which the game has drawn the "Llama Bomb" property, which happens on average once per game year. Once the bombs have been dropped the month continues as usual.
If you wipe out all zombies in a map with the Llama bombs, you will receive some scientists as a reward, only for the zombies to cause another outbreak in the next month.
Ruptured Gas Lines
Even though they hardly qualify as units, the military potential of ruptured gas lines cannot be underestimated. Once unlocked by reaching the right milestone on the Zed victory track, they begin manifesting in the form of specially-marked city buildings. When such a building is destroyed it unleashes a massive explosion, comparable in magnitude and range to a Catbird strike. Although they can be a pain in the neck, especially when it comes to placement of fixed-position units such as artillery or snipers, watch where you blow things up and you'll find them rather useful.
When a mission is completed successfully, your mercenaries are awarded experience. The first promotion is at 125XP, next is at 250XP, with each successive level requiring twice as much.