I was playing with the Bridges, Castles and Bazaars and Die Belargerer (a rerelease of The Siege with 6 tiles).
If you have one in a city, it causes the city to score 1 point per tile and 1 per pennant if it is completed. Now Castles Score what ever the next finished feature in it's 6 tile influence scores.
At the end of game a besieged city is worth 2 cities to a farmer and a castle is worth a +1.
So 3 of the 6 new tiles can be used to create the 'clam' cities you can use for castles.
The way we played is that by making it a castle, it is a castle and ignore the Siege tile from then on.
Is this the correct way to interpret the rules?