A unit is an individual attacking force in Kingdom Conquest. Each base has it's own separate set of units. The number of units you control are determined by your Monster Training Tower. You may build one Monster Training Tower per base, and each one can open as many as five units for that base.
The three slots in a unit are the Leader, Rear, and Front slots. By default, you can only use the Front and Leader slots. The Rear slot is not unlocked for use until you construct a Battle Research Lab. A unit cannot be deployed without a card in it's Leader slot. In battle, the unit is defeated if all monsters in the Leader slot are defeated, even if there are still monsters in the Rear or Front slots.
Units are made up of monster cards. You can place as little as one monster card, and as much as three monster cards per unit. Once a monster card is assigned to a unit, you may produce more monsters for that card. Each monster card can produce up to 9999 monsters. If a monster card has 100 monsters or less, it can be assigned and unassigned to a unit slot without issues. If it contains more than 100 monsters, it can only be transferred between unit slots within the base it is assigned to. If you remove it from an assigned unit (which will then allow you to assign it to a unit in another base), the monster count will be reset to 100. This is an important rule that you must consider before producing large quantities of monsters.
When all of the monsters in a unit are from the same race, a race bonus is issued. This bonus is comprised of two separate bonuses; the research lab bonus, and the hidden bonus.
The research lab bonus is determined by the level of that race's research lab. For example, if your unit is made up of giants, and your Giant Research Lab is LV4, you will receive a research lab bonus of 4%. The hidden bonus is determined by how many monster cards you have in your unit. There is no bonus for a single monster in a unit. If there are two monsters in a unit, you will receive a 5% bonus. If there are three monsters in your unit, you will receive a 10% bonus.
These two bonuses are added together to determine your final bonus. For example, if you have demihumans in your unit, and your Demihuman Research Lab is LV3, you will receive 3% plus an additional 10% totaling 13%. This bonus increases attack, defense, intelligence, speed, and siege of the unit.