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-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.
You can complete an Arena without breaking your Vacation.