It was capital of the Normannic Sector of the Galactic Empire, and once one of its richest planets. Shortly after 100 F.E., Wiscard, the viceroy of Siwenna, rebelled. Most of the subjects remained loyal to the Empire, and overthrew Wiscard, led by Patrician Onum Barr. The Imperial Admiral dispatched to Siwenna was angered at this, because it robbed him of his glory. So, he put most of the population of Siwenna under the atom blast, charging them with the crime of rebelling against an Imperial viceroy (Wiscard). Much of its population was killed, and Barr himself lost five sons and a daughter; only his sixth son, Ducem Barr, survived.
Because of the rampant destruction on Siwenna, the Admiral set himself up as viceroy, but moved the capital of the Normannic Sector to Orsha II. Between this time and its conquest by Bel Riose in 200 F.E., Siwenna rebelled five times, eventually becoming independent. When the campaign led by Riose against the Foundation ended, the Siwenna province transferred to the Foundation, the first Imperial province to pass directly from the Empire to the Foundation.
After the beginning of the Foundation Era, Siwenna began to run downhill. 'The physical resources of twenty-five first rank planets take a long time to use up. Compared to the wealth of the last century, though, we have gone a long way downhill—and there is no sign of turning, not yet,' –Onum Barr, to Hober Mallow, 150 F.E. About 50 years before, Stanel VI died, ending a reign under which Siwenna came close to achieving its ancient prosperity.
Little is known of Siwennian culture, except that when Riose first met Ducem Barr, it was 'socially impossible not to drink tea on Siwenna.'
Consistent with the Roman Empire-Galactic Empire parallelism of the Foundation trilogy, the planet's name may be intended to evoke Ravenna, the city which was the seat of Byzantine hegemony in Italy in the 5th century B.C.