What I am doing as Seleucia:

Number 1, get rid of Egypt ASAP. They will hit you with a non-stop barrage of troops until they are gone. They are also richer than you, so a battle of attrition is one you can't win. Leave just enough troops up north to fend off your neighbors, all your other troops head south to get rid of Egypt.

Number 2, grow Antioch's population as fast as you can. You need phalanx pikemen, and (though I haven't gotten to them yet) elephants.

Number 3, and it looks like you are doing this, roads roads roads. I guarantee before long every single one of your neighbors are going to attack you. You need to be able to move around your armies every turn to counter them.

Also, you will learn to love Militia Cavalry. Disorganize the enemy with your javelins, while your pikemen take care of their scattered forces. Also, when you fight HA heavy armies, you and Militia Cavalry will become best friends. I'm also scooping up as many Cretian Archers as I can, as Seleucia's ranged troops are quite mediocre.