Although Budapest is administratively divided into 23 numbered districts which are always written in Roman numerals, it can most simply be divided into the two cities of which it is comprised (Buda and Pest) and one historic district:
Buda - The hilly West side of the Danube (Districts I-III, XI-XII, XXII).
Castle Hill - District I of Buda, the oldest part of the city containing the eponymous Castle and many of Budapest's best-known attractions (Fishermen's Bastion, Labyrinth and Mathias Church)
Pest - The flat East side of the Danube, covering the modern commercial core of the city(Districts IV-IX).