Castries is the capital and largest city of Saint Lucia, an island country in the Caribbean. The urban area has a population of approximately 20,…Castries is the capital and largest city of Saint Lucia, an island country in the Caribbean. The urban area has a population of approximately 20,000, while the eponymous district has a population of just under 70,000, as at May 2013. The city covers 80 km².