It really depends on what type of player you are. I personally don't consider Arco much of a 'defense' nation. They have a powerful offense in Elephants, Hoplites and Hypsists, as well as great access to spells.

They have the ability to reinforce fairly easily with Astrologers and Gateway. So it's possible to have a large peacekeeping force (with Leadership Items) that can gateway to reinforce your lines.

Also a favorite strategy is to teleport in an Astrologer, cast Doom, then either let him die or have him cast Returning. It's a little high on the chain, but something worth noting for such cheap Astrologers.

The Priestesses allow you to send your SC's into potentially dangerous situations where they might get a few afflictions, and come out and go to the hospital and heal back up. Where other nations do not usually have that option.

Cheap Research, Mystics with multiple paths and a good search/gem income rounds out Arco very nicely.