Yes it is open. The tour prices vary depending on the one that you pick. It is cheaper if you are a member of The Trustees of the Reservation. I suggest that you Google Castle Hill to see the choices
The grounds of the estate are open year round and free to walk. Parking is $15 if not a member $5 if you are a member. House opens in mid April first floor is free to walk around tours cost again on average $15-20 non members $5-10 if you have are a member depending on the tour. The cupola tour is my favorite since you can go on the roof and see the grounds the ocean and for miles on a clear day.
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