, and one of Istanbul's most iconic structures, is the Hagia Sophia. Topped by a dome 31 meters (102 ft) in diameter, the Hagia Sofia stood as the largest cathedral for more than a thousand years, before being converted into a mosque and, now, a museum.Among the oldest extant examples of Ottoman architecture in Istanbul are the Anadoluhisar and Rumelihisar fortresses, which helped block sea traffic aimed at assisting the Byzantines during the Turkish siege of the city.Over the next four centuries, the Ottomans continued to make an indelible impression on the skyline of Istanbul, building towering mosques and ornate palaces. These grand imperial mosques include Sultan Ahmed Mosque (the Blue Mosque), Süleymaniye Mosque, and Yeni Mosque, all of which were built at the peak of the Ottoman Empire, in the 16th and 17th centuries.In the following centuries, and especially after the Tanzimat reforms, Ottoman architecture was supplanted by European styles. In contrast to the traditional elements of Topkap Palace and the mosques on the historic peninsula, Dolmabahçe Palace, Yldz Palace, and Ortaköy Mosque in Beikta, and Beylerbeyi Palace across the Bosphorus in Üsküdar are clearly of Neo-Baroque style. At the same time, the areas around stiklal Avenue were filled with grandiose European embassies and rows of buildings in European (mostly Neoclassical and, later, Art Nouveau) style started to appear along the avenue.