As fortresses are created out of 5 castles, you will also need five times the regular amount of silver in order to conquer another fortress. That also means that if you should conquer a fortress and meanwhile launch a silver carrying attack on a second fortress, you will need 25,000 silver more on the second fortress than you have needed for the first one. In general you can use the following values to help you calculate the correct numbers:
A new castle means that the next castle will cost 1,000 silver, the next fortress will cost 5,000 silver, the next city will cost 25,000 silver and the next metropolis will cost 75,000 silver more.
A new fortress implies that the next castle will cost 5,000 silver, the next fortress will cost 25,000 silver, the next city will cost 125,000 silver and the next metropolis will cost 375,000 silver more.
A new city means that the next castle will cost 25,000 silver, the next fortress will cost 125,000 silver, the next city will cost 625,000 silver and the next metropolis will cost 1,875,000 silver more.
A new metroplis means that the next castle will cost 75,000 silver, the next fortress will cost 375,000 silver, the next city will cost 1,875,000 silver and the next metropolis will cost 5,625,000 silver more.