Notice the stone in the shape of the number zero, placed in the square. This stone is used at the official location where distances are measured from - from Budapest around Hungary and Europe.
From Clark Adam Square board the funicular train (Budavari Siklo) to the top of Buda Hill, or Castle Hill, where the King's Castle is located. This is an easy way to climb the 260 meters to the top of the hill, instead of taking the stairs. Don't want to use the electric train? You can catch the bus, number 26, to the top of the hill.