Budapest is the only Capital in the world with show caves right in the middle of the city! The Danube flows to the east, to the Black Sea, but before it reaches Budapest it makes a turn to the south and flows to the southern border of Hungary, where it turns east again. The city Buapest is actually a unification of three cities, Buda, Pest, and Óbuda (Alt-Ofen). Pest is the town east of the Danube in the plains. The western side of the Danube is hill country with several limestone hills and Buda and Óbuda are located here. And its also where all the caves are located. The 1873 the unification created a city named Pest-Buda or Budapest, which became the Capital of Hungary.
There are numerous hills in the center of Budapest. The limestone and dolomite of those hills was formed in the Eocene, 34 to 55 Million years ago. The caves were formed by thermal water, thus the solution was very fast, much faster than in cold water caves. Also this water dissolved minerals while it was heated up deep below by volcanism, and those minerals precipitated in the caves and formed extraordinary speolethems. The limestone in the water was deposited as aragonite. Typically precipitation from cold water forms calcite, while precipitation from warm water forms aragonite. There are numerous thermal springs which caused the construction of numerous spas. Some spas are inside 19th century hotels and accessible if you are a guest. All those springs emerge from caves, some air filled, some water filled.
The castle stands on Castle Hill, erected in the 11th century. There are natural caves which were integrated into the construction of the fortification. Gellért Hill to the south had a huge cavern in its southern side. In 1926 a lovely chapel was built inside this cave of Gellért Hill, which is today known as Gellért-hegyi sziklatemplom (Cave Church at Gellért Hill) or simply Rock Chapel. The thermal spring of the cave is used in Gellért Spa which is part of Gellért Hotel. All the show caves are in a part of Budapest called Rose Hill, north of Castle Hill. The highest point of Budapest is János Hill, at 527m asl while the lowest point is the shore of the Danube at 96m asl.
Cvaes were used in Buda in multiple ways, show caves, wine cellars, casemates, air raid shelters, and spas. The hot water from the springs is used in numerous spas all over the city center, but some caves are even used for bathing with electric light and trails installed inside the caves. Some of the old hotels have their own cave with a hot spring in the basement.