its basically to on my todo list for future patch yea. high level forts are too easy and low levels are a bit useless and expensive
It is becoming a major issue in multiplayer play. The cost and time to build high levels of fortification is far too little considering the effect. Level 10 forts along the lines of Eben Email or Maginot fortifications took years to build at great cost. In game building high level forts is a trivial effort.
The idea mentioned above making fortification cost and or time to create increase the more levels of fortifications you have should be strongly considered asap.