How to beat horse archers in real life?

[Romano-Dacis]

RTW is rather limited in this regard, but I believe the pavise was a good start to defeating horse-archers. I recall the Persians actually made formations where one man wielded a bow and hid behind another man holding a shield. That is the single best tactic to deal with horse-archers: fortify and shoot. If you have equal or better bows, position archers directly behind shield-bearers. If you have inferior archers, have the infantry fortify themselves as solidly as possible, form an orb around siege weapons and have those weapons take care of it. If you lack siege weapons and archers, the most you can do is slowly retreat to a forest and wait for them to come to you.

cmacq however got it right with terrain. Also, forests really help.