The problem of fort types

[Carkaton]

How about fort upgrades? Take c'tis swamp city for 1200 gold 5 or 6 months build time(can't remember). If they had the option to build the swamp fort initially for 800 and 3 months, with the possibility of upgrading to the city if desired for another 400-600 and more build time.

This would allow nations to eventually make their good forts but keep their competitiveness short term.