When I was playing the Seleucids and out of no where the Pontics and Armenians attack me.
I was getting tired of losing Cretian archers and Rhodian slingers to cavalry who got around my phalanx.
I know this can be countered by my cavalry but at the start of the campaign, I'm a bit tight on cash. And I using my cavalry to do the close hand fighting at the start.
So I started trying this tactic.
I get my phalanx units form a square, 2 units minimum at each side. All the phalanx units work with this, but I recommend at least Levy pikemen. Place the missile units inside. This works great in easy, fairly well in both medium and hard.
I kill alot cavalry with this tactic.