The Sareer: the last desert on Arrakis and the ground of Leto II's Citadel

The Sareer was the last desert on Arrakis during the reign of Leto Atreides II, and the site he chose for his fortress, the Citadel. One desert was left on a planet that had been nothing but desert, and the God-Emperor put his walls around it. (God Emperor of Dune)

How big was the Sareer?#

The Sareer ran 1,500 kilometres by 500, giving an area of 750,000 square kilometres, a shade bigger than the Empty Quarter of the Arabian Peninsula. (God Emperor of Dune)

That is a large piece of ground to hold in place on a world that had otherwise been made temperate. Leto II said as much himself: a river now carried the Idaho name where a mountain had once stood, and the mountain no longer existed, because it had been brought down to supply the high walls that girdle the Sareer. (God Emperor of Dune)

Getting in#

Anyone approaching the Citadel had to pass three obstacles in order, and the desert was the last of them rather than the first. (God Emperor of Dune)

The Forbidden Forest came first, and it was stocked with D-wolves bred to kill whatever entered. The Idaho River came next, crossed by a single bridge that ran to Onn, the festival city that had been Arrakeen. Then came the barrier wall, sheer enough that Duncan Idaho had to climb it by feel, groping for handholds he could not see. Inside all of it sat the sand, and the Citadel on the sand. (God Emperor of Dune)

What happened to the Sareer after Leto II died?#

The Citadel was pulled apart in the Famine Times that followed the God-Emperor's death, by people hunting the enormous spice hoard he was rumoured to have kept there. (Heretics of Dune)

Nothing in the record says the hoard was ever found. What it does say is that the fortress came apart, which means the walls raised out of a whole mountain outlasted their builder by less than the length of a famine. (Heretics of Dune)