King William II of England invades Normandy and gains a foothold in it.
Ladislaus I of Hungary occupies Slavonia.
King Stjepan II of Croatia, the last member of the Trpimirović dynasty, died peacefully without leaving an heir.
October 17 – London Tornado of 1091: A T8/F4 tornado is recorded in St Mary-le-Bow of London, England; the storm destroys London Bridge.
Henry, the third son of William the Conqueror, is forced to surrender his property of Cotentin in Normandy after his two older brothers, William Rufus and Robert Curthose, make a peace agreement.
King Malcolm III of Scotland makes an unsuccessful attempt to invade English territory, but is finally forced to pay homage to King William II of England.
Cardiff Castle is built.
April 29 – Battle of Levounion: The Pechenegs besiege Constantinople, but are defeated so decisively by Emperor Alexius I that they fade into oblivion.
February; With the taking of Noto, the Normans complete the 30-year-long conquest of Sicily from the Islamic rulers.
The Islamic Abbadid dynasty ruling in Spain falls when the Almoravids storm Seville. Confronted with this new threat, the king of Badajoz, al-Mutawakkil ibn al-Aftas, obtains the support of Castile in exchange for the Muslim positions on the Tagus river — Sintra, Santarém and Lisbon.
Roger Guiscard takes Malta.
Athanasius VI bar Khamoro becomes Syriac Orthodox Patriarch of Antioch.
June 17 – Dirk V, Count of Holland
July 5 – Abbot William of Hirsau
August 25 – Sisnando Davides, military leader
December 19 – Adelaide of Susa, margravine of Turin
Robert D'Oyly, first Lord of Oxford Castle
Helena of Hungary, Queen of Croatia
Year 1091 (MXCI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.
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