I hate the parthians as enemies. Mainly because most of their infantry take forever to take down and in the same time you get butchered by arrows.
When you play on huge, armies of horse archers can be easily surrounded by 6-8 wide strechted phalangitai and maybe 1 or 2 light cavallery.
But then there is the parthian infantry... :( Anyway in my roman campain I just let them have the eastern part of the map and I defended all the river crossings against them.
PS: I love too overkill the sarmatians with thracian podromoi. There are sooooooooooo fast and have a merciless charge. In the east there are the indogermanic cavallery, but you have to conquer the province east of bactria^^
Bookmarks