Oman, but which one? After more than an hour of search on the Internet, and maybe hundreds of photos of forts in the sultanate seen, I concluded that is the Nakhal Fort. I wasn't sure, so I did another check and my belief has changed: it's the castle of Samail. I think this in proportion of 80%. Anyway, isn't an UNESCO World Heritage Site, as Cresalde says.
The Samail Gap (spelled also Sumail Gap) is the only natural pass between the Eastern and Western Hajar mountain ranges, so has always been the key strategic link between the interior of the country, dominated by the town of Nizwa, and the coastal plain, dominated by Muscat. Of course, controlling it was vital to anyone with pretentions to rule. Through here passed the old Silk Route, along which traveled the caravans with goods from the Far East. Through here passed Marco Polo and Ibn Battuta.
Although it's now graced by a modern highway, this historic importance is evident in the forts and watchtowers that litter the route. Towns like Fanja, Bid Bid, Samail and Al Khobar are ringed by walls and linked by bridges over rocky wadis that turn into racing torrents when rain falls. Samail has a number of forts and castles, including the Samail Hosn (the castle of Samail) the Bait Al Khubar, the Al Shahbaa Fort and a large number of small forts and Watch Towers. Samail is also famous for its wadi, considered the biggest in Oman, and for its date palm trees which cover most of its area, as can be seen in the picture.
About the stamps
The two stamps on the postcard are definitives from a set of four, about which I wrote here.
Samail - Wikipedia
Sumail Gap - Hampton Court Travel
sender: Cresalde Jumbas Victoriano (direct swap)
sent from Muscat (Oman), on 03.11.2012