Prior to the arrival of the Atreides on Arrakis, the Emperor's right-hand man Count Hasimir Fenring and his wife Margot had resided in the palace. Leto I had chosen Arrakeen for his seat of government because it was small, easier to sterilize and defend.
Arrakeen would go through multiple transformations over time; it first became an Imperial capital of staggering proportions under Paul Muad'Dib. It was later transformed into a festival city known as Onn, explicitly for the worship of the God-Emperor Leto II. Finally, in the centuries after his death, it was known as Keen, a modern (though still impressive) city to house the Priesthood of Rakis.
During the time of the Faufreluches, the city was where the ruling family maintained its household. After the death of Leto II, the city fell under the control of the Priesthood whose deity was the God-Emperor. Even though the Imperium was no more, the city remained an important political centre due to its concentration of those who worshipped the God-Emperor, as well as acting as the administration centre for spice mining operations.
It was located near the northern pole of the planet, and sat largely on a mass of bedrock. This, in conjunction with the protection afforded by the Shield Wall, prevented any incursions from sandworms.
Aside from providing the governing influence with a base of operations, the chief activities in Arrakeen were trade with off-world groups and individuals, and as a gathering point for some Fremen who preferred a more 'cultured' or exotic life.
During the time of the Lost Ones were returning from the Scattering, the city's name had become shortened to Keen.
Duke Leto inspecting Arrakeen spice silos and spice refinery (Dune, 2021)
Thufir Hawat with parasol at spice silos inspection (Dune, 2021)
Spice depot at Arrakeen during dusk, shortly before Fremen sabotage attack (Dune: Part Two, 2024)
Padishah-emperor's personal Starship Lighter hovering above Harkonnen-held Arrakeen (Dune: Part Two, 2024)
The padishah-emperor's personal Starship Lighter arriving outside of the city of Arrakeen and deploying the Emperor's Hutment beneath it (Dune: Part Two, 2024)
The padishah-emperor's Starship Lighter hovering above the deployed Emperor's Hutment beneath it, landed on the plain at Arrakeen (Dune: Part Two, 2024)