Battle Nations Wiki

For the arena we have to distinguish two cases

1. During Arena Challenges
During Arena Challenge one looses points if the defense is defeated. So this has to be avoided by a good and versatile defense. There's no ideal defense as this depends on the own units, their ranks and the player level.
2. No Arena Challenge ongoing
You win a small amount of gold if your defense wins. And you loose nothing if your defense looses. Nobody will attack you if you have a strong defense - nothing to win for you and the others. But you can profit from the arena by attacking defenses with drop formations:

I use drop formations myself for my farming accounts and outside of arena challenges on my main account.

Winning a arena fight gives an additional xp and gold reward. The gold reward is about 30k at level 40 and about 55k on level 70.