Castle of Good Hope
This squat fortress is the oldest still-standing colonial building in South Africa. Built between 1665 and 1676 by the Dutch East India Company (VOC) to replace an earthen fort constructed in 1652 by Jan van Riebeeck, the Dutch commander who settled Cape Town. Its pentagonal plan, with a diamond-shaped bastion at each corner, is typical of the Old Netherlands defensive system adopted in the early 1600s. The design enabled covering fire for every portion of the castle. As an added protection, the whole fortification was surrounded by a moat—back in the day, the sea nearly washed up against its walls. The castle served as both the VOC headquarters and the official governor's residence and still houses the regional headquarters of the National Defence Force. Despite the bellicose origins of the castle, shots have never been fired from its ramparts, except ceremonially.
You can wander around on your own, or join one of the informative guided tours at no extra cost. There are three museums to explore, and you can witness the so-called "Key Ceremony" (10 am and noon, weekdays only) and see a small cannon being fired (10 am, 11 am, and noon, Monday–Saturday).