The Arena is arguably one of the best---and safest---ways to level up Heroes in Imperial Warfare. Unlike Grounds Hunting, World Map Hunting, or Lair Battling, Arena Battles are fought against other players rather than NPCs. As Arena Battles are friendly competitions---even those against Heroes of other civilizations---the damage a Hero takes in the contest is not permanent; moreover, the troops "lost" in battle are restored immediately after completion. It is still in a player's interest, however, to fight Arena Battles with all of his or her might, because only to the victor go the spoils---of experience points (Arena Battle losers gain 10,000 Exp per fight, but as Heroes increase in level this amount seems increasingly pitiful). Moreover, each Arena battle costs 1000 Gold, and it would be a shame to throw away those hard-earned tax bullions just to shame yourself in front of a possible future adversary.
In addition to offering mucho experience upon each victory, Arena battles also contain another source of huge points: Win streaks. Extra experience is given for the fifth, tenth, and every tenth consecutive win after that; the payout offered by this system is so huge that Double Experience books and Experience Crystals don't apply to Arena Battles.