You're not alone in the world, the native inhabitants of this land are Masklings. Masklings are small, psychopathic creatures notable for their red hoods, white masks, and hatred of everything you love. They live in camps scattered around the world, and will launch periodic raids at your settlements.
Individual Masklings are fairly weak, easy to kill, and armed only with spears. Their strength comes from numbers.
Masklings know of each of your settlements, and remember how difficult they are to attack. Periodically (see Time for specifics) they will launch a raid. When launching a raid they pick one of your settlements to be the target.
The first stage of a raid is scouting. A scouting party will consist of one or to Masklings, who will approach one of your settlements to examine it before running back home.
You can intercept and kill the scouting party before they reach your settlement. If you do, then the raid will be delayed as another scouting party will be sent instead. If the scouts manage to reach your settlement, however, then a raid will be launched.
The next step in raiding is to muster forces. Masklings form warbands from nearby camps, up to a limit. The limit is based on the settlement being targeted. The tribe remembers how difficult the last raid was, and plans accordingly.
The warbands will approach the target settlement, waiting nearby until all of the warbands are in range. Once all of the warbands are near the target, they start the assault.
All of the warbands attack the target settlement at once, charging straight at the center of it and destroying any structure in their way. They will prioritize nearby enemy warbands over structures, allowing you to send your troops to hold off the assault. Each citizen they kill and structure they destroy are counted towards their "Kill Limit". Once that kill limit has been reached they will retreat back to their camps. They will still attack any citizens and structures they encounter, even when retreating.
Depending on how the raid ends, the Masklings will adjust how they approach the next raid against that settlement.
If the raid was defeated (every Maskling killed before they reached their "Kill Limit") the next raid will try to bring more warriors, up to a limit of 3 x the settlement's population.
If the raid succeeded (the Masklings reached their "Kill Limit") they will increase the "Kill Limit" instead, aiming to do more damage next time.
The most direct method of countering raids is to defeat them. Walls will slow down their advance, while Towers and Gatehouses will attack and kill them.
Another method is to counterattack by forming your own warbands. You can send your own forces to attack the Maskling raiders, or attack their camps directly. Each camp destroyed will reduce the intensity of all raids from the Maskling tribe.
A race of mole-like creatures that dig underneath the ground (and presumably under your defences). Their burrowing is very quiet leading to little notification of an impending siege, until it's too late. They also have smaller pets and mounds scattered around the world that serve as entrances to their colonies (unknown why, as they just travel via digging).
Tanky, Oger creatures. Presumably, highly resistant to arrows and strong enough to make quick work of your walls, and your soldiers.
Nothing is known about this race, minus the name. Although considering it's name, it's most likely a swamp-dwelling species, and if the addition of "people" is anything to go by, it could quite possibly be an intelligent species, possibly focusing less on blunt attacks, and more on war machines (i.e. Trebushes) and high-end soldiers (possibly meant to be a late-game counter to 100+ men armies of Spearmen and Archers).
Each biome is planed to have unique and small monsters to pick off wandering citizens and keep your war-bands on edge. Grasslands may have Wolves, Forests may have giant spiders, and so on.