Maruhubi Palace was built outside Zanzibar Town in 1882 for Sultan Barghash to house his impressively large harem. A few years later it was destroyed by fire, although the remaining walls and arches, and the large columns that once supported an upper balcony, hint at its previous scale. The entrance is 4km north of Zanzibar Town, on the left (west) of the main road towards Bububu.
The guide (included in the entrance fee) has old photos of the palace shortly after it was built; it's fascinating to compare these with the view today.