Kota Batu, meaning "Stone Fort," is an archaeological site a few kilometres east of Bandar Seri Begawan that served as Brunei's ancient capital from the 14th to 17th centuries. The site was the heart of the Brunei Sultanate during its golden age, when the empire extended across much of Borneo and the Philippines. Today it's a quiet place of excavated ruins, old Chinese trade ceramics, and a small museum cluster that makes a good half-day excursion from the capital.
The Brunei Museum holds artefacts recovered from the Kota Batu Archaeological Park — Islamic ceramics, coins, and everyday objects from the sultanate period — and provides essential context for understanding what you're looking at among the ruins. The Brunei Maritime Museum next door focuses on the sea-trading heritage, including an extraordinary recovered cargo from a 15th-century shipwreck. The adjacent Malay Technology Museum covers traditional crafts and is worth a quick stop.