Whether you'd like to pray or find out about the history and background of the building, St Marys on the Harbour is a worthy site to explore. Check to see if this holy place is publicly accessible to experience the calmness inside.
You'll have no end of sightseeing choices when in Castletown. St Mary's on the Harbour is a good place to start here, but there are many other interesting attractions to consider. Port Erin Beach, Castle Rushen, and Spanish Head are not far from the house of prayer and will round out your day nicely.
