Some territories of Warzone Idle may have an Arena. An Arena lets you get bonus resources (armies, money, ores, etc.) by playing a Quickmatch game on a fixed template. You can play the Arena game upon conquering the Arena territory or defer it for later (by closing the pop-up).
Like with mines, smelters, caches, and crafters, Arenas spawn on fixed locations. The template for an Arena game is also pre-determined; it's listed in the Arena pop-up (visible upon either conquering the territory or clicking the "Play" button in the territory description).
Arena games are ordinary Quickmatch games (except with a specific template). They will affect your Quickmatch rating.
 Reward & Redemption
The Arena reward is essentially a double sized cache.
It is redeemed upon returning to Idle after winning a Quickmatch Arena game. You can retry the Arena as many times as it takes for you to win. The Arena will disappear once you claim its reward.