Complex sentence: In the first world war the Mere was the briefly site of "RNAS Hornsea", a seaplane base; during the second world war the town and beach was heavily fortified against invasion.
Split into simpler sentences: In the First World War the Mere was the briefly the site of "RNAS Hornsea", a seaplane base. During the Second World War the town and beach was heavily fortified against invasion.