Óbuda is the oldest of the three towns that became Budapest, and it wears its age strangely: a baroque village square marooned among Communist tower blocks, with a Roman city buried at the northern end. The walk runs straight up through two thousand years.
A little under five kilometres, best in the afternoon. From Szentlélek tér, step into Fő tér — cobbles, a yellow mansion, and almost nobody. In a side lane, Imre Varga's four bronze women stand under umbrellas waiting for rain; you can walk right in among them. The Kassák museum, devoted to Hungary's lone avant-gardist, sits in the same baroque courtyard, an odd and pleasing collision.
Walk north to the Roman military amphitheatre — an open oval of turf and stone you can stand in the middle of — then on to Aquincum itself, the civilian capital of Roman Pannonia. Its grid of low walls, baths and streets makes the lost city walkable.
Finish at the Danube the legions guarded, and take the H5 back from the Aquincum stop.
