Dens are a physical object that can be destroyed and that will stop critters from spawning out into the world. If the dens go unchecked, the area will start to crawl with critters, and eventually they will start to grow in size as well as numbers. Critters offer economical value for crafting, like many other games. They provide all sorts of needed components that can be extracted once they are dead. The critters and dens can also be targeted during missions offering PvE elements from the cities and additional resources for crafters and adventurers both.