This is the entire reason you’re visiting Hue. From 1802 to 1945, Hue was the capital of Vietnam and home to the Nguyen dynasty. The Imperial City is a UNESCO World Heritage Site and the citadel was built between 1804 and 1833, is a heavily fortified structure with a moat across it. Make sure you see both the Imperial Enclosure (full of scenic gates) and Purple Forbidden city (after the Tet Offensive and Battle of Hue only 10 original buildings remain). If you can, make it to the Co Ha Gardens, which will make you feel like Vietnamese royalty.
We recommend a guide. You have a few options: Buffalo Tours, Handspan, Tours by Locals, or Footprint.