This castle was built in the mid-18th century in one of Oman's oldest cities - and its capital in the 6th and 7th centuries CE, a drive of about an hour and a half from current capital Muscat. Its main feature is a huge oblong tower rising 30 metres above the ground and 36m around. It's open to visitors and includes museum displays and artisans practising traditional crafts. This YouTube video gives you a good sense of what it's like.
More information: NizwaFort.om