Hello Fiona, I was with a guided tour and it was during the day so opening times were on my mind, but you can safely say that it will be open during reasonable daily hours, but I would check with maybe your hotel as they may know. Also, check when public holidays are as this may affect time too.

Brilliant Way has it nailed - get there early, we did!

Hue Citadel is more than 250km2 wide. You buy entrance tickets directly at each entrance to each destination, according to the general regulations of the local government, except for private spots. Tien Phong newspaper wrote: Tickets to visit Hue Royal Palace (including the Citadel and Museum of Hue Royal Antiquities) cost 200,000 VND/adult and 40,000 VND/child. The monuments of Minh Mang, Tu Duc and Khai Dinh tombs have entrance fees of 150,000 VND/adult/point. The tombs of Gia Long, Thieu Tri, Dong Khanh, Hon Chen Palace, Nam Giao Forum, and An Dinh Palace have a ticket price of 50,000 VND/adult/point. Tickets to visit the route are priced at 420,000 VND, 530,000 VND, 580,000 VND/adult, depending on the number of Hue relics aggregated points.

They have various routes, but I cannot put a link here (not allowed by TA). I suggest #5 90K VND per person to visit the perimeter or outer area (Hoang Thanh) After that, you can walk around the inner area (the Purple City or Dai Noi.) If you still want to take another trip, pay another 100K VND for the 30 minutes of the inner area. ... If you don't want to visit the perimeter, pay 150K for the 60-min ride of the inner area. ... The other routes (1-4) are point-to-point. When you get there, you'll see what I mean.

Sure is, at the ticket counter

I seriously doubt it. And the Citidal is not at all an accessible location.

I seem to remember it was cash only. I found a lot of places in Vietnam would only take cash.

Buy tickets when you are there. No queue and ticket office is outside the Citadel entrance.

Sorry I should have said 17.30 for Tuesday closing time but Thursday it is open till 22.00.

It is a lot of walking, Jolyon. The Citadel walls form a square, about 700 meters (1/2 mile) across, plus a good 5 minutes' walk on each end to the car parks. The various complexes of building are on different sides inside the walls, so you end up walking back/forth, and enter on one side (the Meridian Gate) and exit on the opposite. If there is a plus, it's all flat--just a few steps to get into buildings. When I visited I didn't see any buggies like another person answered, so I can't vouch for that.