[JimMorrison]

I'm afraid a straight conversion would be a bit ridiculous in the early game, and would also confer far too large an advantage to those who CAN make Fortified Cities, as opposed to those whose best castle is 30 Admin, and hard to find a spot for.

People would just push the PD to 30+ at strategic locations, so the completion of the castle would push it over 80, which would be just insane for most nations.

Also bear in mind that just adding PD numerical value also has an enormous disparity between nations, some nations get little of value even at such extreme numbers. Niefelheim would especially be hard hit, with poor PD AND max Admin of 30 in their castles.