Castle of Saint Charles

More fort than castle, Sant Carles was built between 1610 and 1612 on the site of an old lighthouse and now houses the Museu Històric Militar. You'll find fascinating exhibits on Mallorca's coastal defence system, including info on the 86 historic watchtowers that still dot the island's perimeter. It's a surprisingly rich slice of military history with sweeping views over the bay thrown in for free.