Bridges, Castles and Bazaars with Die Belargerer (the Siege)
When using castles, if a 'clam city' is completed you then make the decision to either score it as a city or turn it into a castle. If it becomes a castle, then no scoring relating to cities applies to it (and scoring for the castle does not occur until and unless a neighboring feature is completed). Since the scoring of siege tiles only applies to cities, it does not affect the scoring of a castle. A castle is worth 4 points to a farmer. 

Link to official rules:

http://zmangames.com/rulebooks/Carcassonne_Exp8.pdf