Complex sentence: Because seafood is quick to spoil, fish markets are historically most often found in seaside towns, but (if ice or other simple cooling methods were available) some were also established in large inland cities that had good trade routes to the coast.
Split into simpler sentences: Because seafood is quick to spoil, fish markets are historically most often found in seaside towns. Once ice or other simple cooling methods became available, some were also established in large inland cities that had good trade routes to the coast.