Arrakeen was one of the largest cities on the desert planet Arrakis, and its seat of power.
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.
For thousands of years, Arrakeen had been a relatively primitive settlement built on the Shield Wall, southeast of Carthag.
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 becomes an Imperial capital of staggering proportions under Paul Muad'Dib. It is 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 is 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. The Bene Gesserit, the Fish Speakers, and other groups maintained a presence there in various forms.
When the Honored Matres attacked and destroyed Rakis with their super-weapon Obliterators, Keen was buried under a mountain of slag. Over two decades later, a CHOAM expedition to salvage some of the artifacts from the buried Priesthood complex yielded few rewards, although Guriff and his scavengers stayed behind to keep searching.
After the revelation of the Ultimate Kwisatz Haderach after the Day of Kralizec, the gholas of Paul, and his wife, Chani returned to the area around Keen. Under the guidance of the super-being/Evermind Idaho, Arrakeen and Rakis was being returned to its pre-Obliterator state of existence.