The Citadel was the top layer of Minas Tirith, within the seventh wall. Within the city are the White Tower of Ecthelion, the House of the King, Merethrond (the Feasting Hall) and several other buildings. The Citadel would have been around 500 feet across.
The Citadel is set out this way: In the centre is the White Tower, rising to a great height. Behind is the large sprawling King's House, where the Kings of Gondor once lived. In front of the tower is the Place of the Fountain, a courtyard containing the White tree and a central fountain. Around the Tower are ringed several other important buildings, including Merethrond, the Feasting Hall. The Citadel is accessed via a wide tunnel through the hill that leads to a small gate. The wide cliff narrows to its point around 200 feet beyond the Citadel. As with all the city's levels, the Citadel has a high wall and seven towers.
The Place of the FountainEditProbably the most important symbolic area of the city, the Place of the Fountain was the home of the great White Tree. It was a large circular grass area, around 50 feet wide, with the White Tree being the central feature. The fountain was positioned some way before the tree. It flowed into a small pool, then back round to the fountain. The Tree symbolised Gondor, and was the flag and emblem of the City Guards. The Tree was grown from a seed of Nimloth, the tree given to the Númenóreans by the Eldar. It was planted by Isildur, and grew to symbolise the blessings of Men. But in the third age the tree was withered, perhaps symbolising Minas Tirith's decline. It was only with the crowning of Aragorn Elessar that a new sapling was founded, showing the City's rise to glory under the new King.
The Tower and other buildingsEditThe Tower of Ecthelion is a tall white stone structure, rising 300 feet above the citadel. The elevation of the city means it's topmost floor is 1000 feet above the Pelennor Fields. It's name implies that it was probably built by Denethor's father, Ecthelion, but it was possibly much older. The Tower was used as a main business building, and contained several large council halls, and the imposing Throne Room. At the summit was the small chamber where Denethor kept Minas Tirith's Palantir. At the very bottom of the Tower, there were several rooms for the Guards of the City to live and work in.
The House of Kings was a large building, with many halls and towers. As one of the main buildings in the city, it would have been around 100 feet long. It was designed to accommodate the King and royal family, visitors, guests and a large team of servants. The palace was no longer in use by the time Gandalf and Pippin came to the city; the last King, Eärnur, having long since been killed.
The only other building Tolkien specifically mentioned was Merethrond, the Hall of Feasts. This is where important guests to the city were entertained, and where important feasts and celebrations were held.
There were several other structures on the citadel, ringed around the Tower, but none apart from the White Tower and the Feast Hall are important to the story of lord of the rings.