The Royal Palaces of Abomey certainly were a highlight of my road trip in Benin. I did read about the "no photo" policy but with the help of a few Benin Francs that policy was abandoned very fast. Not with a big camera but with the iPhone I was allowed. Perfect, as the phones make good photos anyway these days.
The palaces are well restored (late 2019) and still some reconstruction was going on. Mainly restoring of the tatched roofs. A visit gives a good look into the Africa before the Europeans came. The tombs are also interesting to visit. The museum has relics from all the kings on display and also shows the history of the kingdoms. There are actually still kings in Benin today but, except some rule over the local community, they have not much to say.
The site is easily combined with a visit to the subterrean housing about 45 minutes away (Village souterrain d'Agongointo-Zoungoudo). A day trip from Cotonou can easily be arranged.