Istanbul is primarily known for its Byzantine and Ottoman architecture, but its buildings reflect the various peoples and empires that have previously ruled the city. Examples of Genoese and Roman architecture remain visible in Istanbul alongside their Ottoman counterparts. While nothing of the architecture of the classical Greek period has survived, Roman architecture has proved to be more durable. Obelisks from the Hippodrome of Constantinople are still visible in Sultanahmet Square, while a section of the Valens Aqueduct, constructed in the late 4th century, stands relatively intact at the western edge of the Fatih district. The Column of Constantine, erected in 330 AD to mark the new Roman capital, still stands not far from the Hippodrome. Early Byzantine architecture followed the classical Roman model of domes and arches, but improved upon these elements, as in the Church of the Saints Sergius and Bacchus. The oldest surviving Byzantine church in Istanbul—albeit in ruins—is the Monastery of Stoudios (later converted into the Imrahor Mosque), which was built in 454. After the recapture of Constantinople in 1261, the Byzantines enlarged two of the most important churches still extant, Chora Church and Pammakaristos Church. Still, the pinnacle of Byzantine architecture, and one of Istanbul's most iconic structures, is the Hagia Sophia. Topped by a dome 31 meters (102 ft) in diameter, the Hagia Sophia stood as the world's largest cathedral for more than a thousand years, before being converted into a mosque and, as it stands now, a museum. Among the oldest surviving examples of Ottoman architecture in Istanbul are the Anadoluhisarı and Rumelihisarı fortresses, which assisted the Ottomans during their siege of the city. Over the next four centuries, the Ottomans proceeded to make an indelible impression on the skyline of Istanbul, building towering mosques and ornate palaces. The largest palace, Topkapı, includes a diverse array of architectural styles, from Baroque inside the Harem, to its Neoclassical style Enderûn Library. The 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. Areas around İstiklal Avenue were filled with grand European embassies and rows of buildings in Neoclassical, Renaissance Revival and Art Nouveau styles, which went on to influence the architecture of a variety of structures in Beyoğlu—including churches, stores, and theaters—and official buildings such as Dolmabahçe Palace.