Castle Acre Priory near Swaffham is preserved Cluniac Priory with a Norman facade.
Quite often has special weekends, look for details in the TICs.
EH admission charges apply.
Web site

Baconsthorpe Castle is near to Sheringham an has brown signs on the A 148 directing you to it.
It is not a castle but the ruins of a fortified manor house complete with moat and lake.
There is a car park and NO admission charge. Large green areas enable you to relax and enjoy a picnic amidst the wildlife.
Note, there are no toilet facilities.
Web site

Castle Rising Castle on the A149 past King’s Lynn is an imposing keep set in 12 acres of defensive earthworks.
Car parking is available and gift shop and toilets.
Web site

Binham Priory near to Holt is a free entry property and a place where you can enjoy a quiet stroll around the ruins. Well worth a detour to view.
Web site