Spug is a port town in Dalmatia that was threatened by the Turkish pirate Ragbash, an event in the history of the Grand Central Schism that led the pirate to being named a pope (Saint Fidgeta and Other Parodies, 59).
Dalmatia is one of the four historical regions of Croatia, alongside Croatia proper, Slavonia, and Istria. It is a narrow belt of the east shore of the Adriatic Sea with 79 islands (and about 500 islets) running parallel to the coast. The largest city is Split, which Bellairs may have parodied as Spug. The name Dalmatia derives from the name of the Dalmatae tribe, which is connected with the Illyrian word delme meaning "sheep" .