The Fatih district corresponds to what was, until the Ottoman conquest, the whole of the city, across from which stood the Genoese citadel of Galata. Those Genoese fortifications were largely demolished in the 19th century, leaving only the Galata Tower, to make way for northward expansion of the city. Galata is now a part of the Beyoğlu district, which forms Istanbul's commercial and entertainment center around Taksim Square. Dolmabahçe Palace, the seat of government during the late Ottoman period, is located in Beşiktaş, just north of Beyoğlu, across from BJK İnönü Stadium, home to Turkey's oldest sports club. The former village of Ortaköy is situated within Beşiktaş and provides its name to the Ortaköy Mosque, along the Bosphorus near the First Bosphorus Bridge. Lining the shores of the Bosphorus north of there are yalıs, luxurious chalet mansions originally built by 19th-century aristocrats and elites as summer homes. Farther inland, outside the city's inner ring road, are Levent and Maslak, Istanbul's primary economic centers. During the Ottoman period, Üsküdar and Kadıköy were outside the scope of urban Istanbul, serving as tranquil outposts with seaside yalıs and gardens. However, during the second half of the 20th century, the Asian side experienced massive urban growth; the late development of this part of the city led to better infrastructure and tidier urban planning when compared with most other residential areas in the city. Much of the Asian side of the Bosphorus functions as a suburb of the economic and commercial centers in European Istanbul, accounting for a third of the city's population but only a quarter of its employment. As a result of Istanbul's exponential growth during the 20th century, a significant portion of the city is composed of gecekondular (literally "built overnight"), referring to illegally constructed squatter buildings. At present, some gecekondu areas are being gradually demolished and replaced by modern mass-housing compounds. Moreover, large scale gentrification and urban renewal projects have been taking place, such as the one in Tarlabasi; however, some of these projects -like the one in Sulukule- have faced criticism. The Turkish government also has ambitious plans for an expansion of the city west and northwards on the European side in conjunction with plans for a third airport and the city's Olympic bid; the new parts of the city will include four different settlements with specified urban functions, housing 1.5 million people. Istanbul does not have a primary urban park, unlike other large cities, but it does have a number of green areas. Gülhane Park and Yıldız Park were originally included within the grounds of two of Istanbul's palaces—Topkapı Palace and Yıldız Palace—but they were repurposed as public parks in the early decades of the Turkish Republic. Another park, Fethi Paşa Korusu, is situated on a hillside adjacent to the Bosphorus Bridge in Anatolia, opposite Yıldız Palace. Along the European side, and closer to the Fatih Sultan Mehmet Bridge, is Emirgan Park; originally a private estate belonging to Ottoman leaders, the 47-hectare (120-acre) park is known for its diversity of plants and an annual tulip festival held since 2005. Popular during the summer among Istanbulites is Belgrad Forest, spreading across 5,500 hectares (14,000 acres) at the northern edge of the city. The forest originally supplied water to the city and remnants of reservoirs used during Byzantine and Ottoman times still survive. 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.