The bare peak of Sas (means Eagle) Hill on the edge of District 11 and 12 is a natural reserve in the middle of the city. The 265 meters high double peaked dolomite hill was formed around 200 million years ago. Erosion has resulted in more rock formations of special shapes such as Bear Rock and Beethoven Rock. The 30 hectare upper part of the hill gained a protected status in 1958.

The hill bore several names throughout the centuries. It was called Muhanek during the Turkish rule, Adelsberg at the end of the 19th century, which probably later changed to Adlersberg (Eagle’s Mount). According to a legend, when Buda was retaken from the Turkish in 1685, eagles symbolizing freedom flew towards the castle from the hill thus giving its name to it.

Until the Philloxera epidemic vineyards spread up the hill. The first villas on the slopes appeared in the mid-19th century. By the end of the century the Saxlehner family was among the owners of the largest areas on Sas Hill. They planted lilac there, which, despite its scent and beauty, badly damages its environment by killing nearby plants and harmfully affecting soil. This is still a challenge in sustaining the natural reserve.

The natural reserve area belongs to Duna-Ipoly National Park. An exhibition entitled “A shelter in the concrete jungle” focuses on the history of Sas Hill also presenting local fauna and flora. The visitor center provides interesting guided tours along the nature trail between February and November in English too. Another trail leads directly to a lookout spot. The trail is paved and wheelchair accessible.

The most common plants of the hill include hawthorn, blackthorn, lilac and downy oak. The temperature variation on the two slopes of the hill allows to observe plants of different micro climates. Occasionally, temperature on the bare southern slope is 15-20 °C higher than on the bushy northern side. The southern slope provides habitat only for green spots on the bare dolomite rocks. One of the survivors here is seseli leucosper, which especially prefers dolomite environment. It grows only in Hungary. Another species of the hill is the specially protected joint-pine with only male plants on Sas Hill and female ones on Gellért Hill. They are wind pollinated. However, the northern slope is rich in vegetation and a special species there is the Hungarian blue grass.

Sas Hill gives shelter to numerous animal species as well, such as various lizards, 170 kinds of spiders, butterflies, Caspian whipsnakes, European mantis, sparrowhawks and common buzzards. Sas Hill is attended by kestrels also to feed and learn to fly.

A bird observatory, a vineyard and a herb garden are each planned for the future as well as a fruit garden on the southern slope to grow native species of peach, plum, cherry and nut.

The north-eastern slope of Sas Hill is home to the seismological observatory of the Hungarian Academy of Sciences specializing in detection and research of earthquakes. The best known building of the area is the yellow colored building of János Arany Primary School and High School. It was built for the Congregation of Our Lady of Sion in 1930 by the design of Gusztáv Szörényi–Reischl. In 1948, schools were nationalized and a year later nuns left the school and the country. The same year a school named after the Soviet hero teenager, Oleg Kosevoj moved in the building. The school prepared scholarship students for university studies in the Soviet Union. When it was closed in 1954 a new school was established under the name of writer János Arany.