Castletown, Isle of Man offers 4 beaches perfect for sunbathing, surfing, swimming, and more. Castletown, Isle Of Man, Isle of Man, is known for its historic Castle Rushen, a well-preserved medieval fortress. The best time to visit Castletown, Isle of Man, is from June to August, with average air temperatures around 15-18°C (59-64°F) and sea temperatures about 13-15°C (55-59°F). Sandee provides detailed maps, photos, and reviews from fellow beachgoers to help you Choose Your Beach™ and plan your perfect beach vacation!