Ok, Fortresses needs towers, but their cost is very well implemented, with mordor you have a enormous economy focused on structures so, you can buy 1 of 2 in mid game, and start getting awesome troops, specially Castellan and Morgul Riders are awesome units, who can change the batle in a few seconds. Morgul melee infantry and pikemen are very hard to kill and dol guldur archers make a enormous damage, so I think 3000 is a good cost for both fortresses. if the edain team decide to give to this fortreses some upgrades I will be agree, because sometimes is easy to the enemie send a few batallions and destroy easily this Fortreses. ( case of Morgul) Dol Guldor have right now a mighty poison damage which make imposible to get it down with light troops.