The Secret faction, as the name implies, is made up completely of "secret" Units. You can unlock these units by finding relevant items on the map and looking at the item for about 5 seconds in free-cam mode or during a battle. This faction seems to be a dump for extra units when there were more than 7 Units assigned to a faction or a place to put Easter eggunits (because it has 39 units). Even though this is most likely its own faction, most of the units here would be able to fit in the other factions (Like the Jester in the Medieval Faction or the Ballooner in the Renaissance Faction), but are either too powerful (such as the Ice Giant and Artemis), or are Units that are too situational in battle to be considered part of the main factions, like the Mace Spinner or the Ball 'n' Chain.
As a special faction that hosts almost every Unit that is found on maps and couldn't fit in the other ones, or are one too many for the faction they could belong to, it doesn't have it's own theme, therefore, it has no map to stick to their theme, although it's maps could be the Simulation maps.
For the same reason, it doesn't respect the 7 units per faction rule, having 39 units in total.
The secret Units are all unlocked by hovering over something in the map, and then unlocking it. The Bank Robbers are the only unit(s) that break/add to this rule, as to unlock the Bank Robbers, you have to break the wall at the back of the bank on the Western Map, then look at the vault for a little bit.
