The tunnel which crosses through Castle Hill was built by the same architect as the Chain Bridge, Adam Clark. Completed in 1856, the 350 meter tunnel remains an important traffic artery.
The tunnel provided a direct link from Pest, over the Chain Bridge, and into Buda. During the seige of Budapest in the Second World War, the Germans baricaded themselves inside and attempted to escape the Soviets by using old underground tunnels. Most of them were captured. Today, while it's possible to walk through the tunnel it isn't recommended - The traffic moving through the tunnel causes quite a bit of stagnant pollution and makes it very difficult to breath.