Know Before You Go
Gillette Castle State Park, which comprises the castle ground and mountainous park land, is open most days from 8am to dusk.
The castle itself is open only on certain days, for about six hours in the middle of the day. Check the website for exact days and hours.
You can purchase tours, which as of the summer of 2015 were about $20 per person, which grant you access to the castle and visitor center, along with an expert tour guide. The interior is as eccentric as William Gillette himself, so an interior tour is not to be missed if you make the trip to the park.