Castle of Santiago was built in the second half of the 15th century by the 2nd Duke of Medina Sidonia. It was not designed for military or surveillance purposes. Therefore, it can be considered as a demonstration of power of the Duchy. Throughout its history, it has functioned as a ducal fortress and barracks. After almost a century of neglect and suffering serious damage from a fire, it was completely remodelled.
References:Herceg Novi was founded (on a former small fishing village, existing since Roman Empire times) as a fortress in 1382 by first Bosnian King Stjepan Tvrtko I and was called Sveti Stefan or Castelnuovo. The Turks conquered Herceg Novi in 1482, and ruled for 200 years, until 1687. However, there was a short pause between 1538 and 1539 when it was held by the Spaniards before they were defeated in the Siege of Castelnuovo. Turkey ruled again until 1687, from then until 1797, the town was ruled by the Venetian Republic.
The Herceg Novi old town is amazing. It is on a fairly steep hill that leads all the way down to the sea. Wandering through the small stairways to the various plazas and fortresses is a many hour adventure.