The smallest and most picturesque of Ibo's three forts, Fort Santo Antonio sits a short walk south of the old town. It is a compact whitewashed bastion still in fairly good repair, and it now houses the workshops of several of the island's silversmiths -- you can wander in, watch the work, and buy a piece on the way out.
The fort was built in the late eighteenth century as part of the Portuguese chain of defences around the Quirimbas trade routes. Climb the short ramparts for a good view across the channel to the mangroves and the dhows working the tide.