Encampments are small military bases that set up after a land tile has been occupied by a player.
Encampments however take up a Marching slot which means once an encampment is made it can only be recalled to free up the marching slot.
An encampment can be used to hide troops away from your city, so that in the event of an attack on your city, the encampment troops are protected.
Some players choose to surround their city or several cities with encampments, this prevents an enemy from teleporting right next to their city. The added distance means attacks take longer to reach the city, giving the player extra time to prepare defenses, shield, or counter attack.
Others have used encampments for tributes or protests with hundreds of encampments surround a single city.