When the plans for the Chain Bridge began to take shape, another idea emerged alongside it:
to create a passage not only across the river but also beneath the hill, linking Pest with Buda.
The concept itself was not new – Dániel Novák had already proposed a crossing in 1837,
originally planned to run under the Danube,
and later, after reconsidering the technical challenges, he suggested a more modest version beneath Castle Hill.

In the 1840s, Count István Széchenyi embraced the project and founded a company to prepare for the construction of the tunnel.
The plan became one of the emblematic undertakings of the Reform Era,
but the outbreak of the 1848–49 Revolution and War of Independence halted the initiative.
It would take several years before the idea could be revived in a new form.

The designer of the Chain Bridge, William Tierney Clark, had already completed the first surveys and draft plans in the 1840s,
but construction did not begin.
After the bridge opened in 1849, József Ürményi, Lord Lieutenant of Pest County,
revived the concept and organised a joint-stock company to carry out the project.
The work was directed by Adam Clark, resident engineer of the Chain Bridge,
who used William Tierney Clark’s earlier plans as a foundation while preparing the final technical drawings
and supervising the construction.

The Vasárnapi Újság (Sunday News) of 27 April 1856 described the new engineering marvel as follows:

“Crossing the Chain Bridge, just a few steps from the foot of the hill
yawns the half-egg-shaped mouth of the tunnel.
Stepping closer to the entrance, a long row of lamps comes into view,
glimmering like sparks at the far exit,
yet flooding the vast vault with light.
Walking from the mouth toward Buda,
the houses of Pest slowly disappear from sight;
the people at the entrance seem tiny,
and soon we find ourselves in the Horváth Garden on the Buda side.
Thus one travels beneath the hill, the castle walls and the royal palace,
and after five minutes reaches Buda’s valley part,
where earlier, even in good weather, half an hour was needed.”

The tunnel measured 180 fathoms long (about 342 m) and 5 fathoms wide (about 9.5 m),
built with stone and brick masonry under a semicircular vault.
Its eastern portal followed a classical, the western a neo-Romanesque style.
Construction was completed by the end of 1856, and the passage opened to traffic in 1857.

The Buda Castle Tunnel and the Chain Bridge are complementary works:
one connects the two sides of the city across the river, the other beneath the hill.
Both express the same intention — to overcome the natural barriers dividing Pest and Buda
and to create connection instead of separation,
just as István Széchenyi envisioned.