I visited the citadel from Alba Iulia on a very hot August day, but not even the 40 degrees Celsius ruined the great experience I had.
Recently restored and conserving history stories and ruins that go back almost 2,000 years, the citadel is one of the finest of this kind in Europe and one of the best attractions in Romania.
You can choose from various thematic itineraries, and combining them is a good option to see all the highlights: the monumental gates, the museums and the ruins of the Roman castrum.
The changing of the guards' ceremonial takes place everyday at 12 pm. Make sure you don't miss it.
Overall, you should take at least 5-6 hours to explore it properly.