IRL, people didn't have to deal with those Pajama Infantry the way you do in RTW however. They charge your towns with maybe...8+ groups of pajama men and 2-3 horses. Then they retreat away after half their force gets slaughtered. Half way out the route, they come back in and repeat this until they are all dead. Takes forever, even with speed on when it looks like ping pong. I'd chase them down, but the usual cav I leave to garrison is the crappy militia cav. Parthia cav eats mine up when I try and kill the routers. Annoying.
That gate open trick with phalanx is how I survived early game as Seleucids in vanilla. Seriously felt like that guy that made his Portugal nation a line from Lisboa to Korea in Eu3. Leaves my defense threadbare. Completely expected my 3-4 Militia Hoplites to hold off hordes and hordes of Pajama-men, Egyptian Naked Fanatics Warriors, Greeks, Romans, and Pontus peeps.
I will have to try this new mod you speak of for RTW, total realism was cool but eh sometimes. The east was simply overcome the Seleucid Empire, because they keep the other factions down in the dirt for you. The West was either take over Italy or Africa and Spain is now completely open for you. After that, you win the game as Seleucids will most likely have crumbled while the other nation split it so evenly they don't stand a chance. (Also, Illyria in RTR is famazing =D )