The best way to explore Nizwa is on foot, to know the culture and history of Oman is to see also the past. You will finde a rich blend of history, culture and nature. It is located around 2 hour drive from Muscat. First through the Nitzwa Fort, built in the 17th century, is one of the most historic landmark in Oman, a must see if you walk along the city. the best time to visit is early in the morning or late afternoon. I think you can find a few forts in every small town in Oman. According to what I read, these historical forts were built as a protection to defend the territory during the troubles and to consolidate peace.
Have a beautiful day!