1091 (MXCI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.
Events
By area
Europe
King William II of England invades Normandy. Ladislaus I of Hungary occupies Slavonia. King Stjepan II of Croatia, the last member of the Trpimirović dynasty, died without leaving anyone to inherit his thrown and title.
British Isles
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 give up 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 give a public statement of allegiance to King William II of England. Cardiff Castle is built.
Mediterranean
April 29 – Battle of Levounion: The Pechenegs surround and attack Constantinople, but are defeated so completely by Emperor Alexius I that they are completely eliminated. February; With the taking of Noto, the Normans complete the 30 year long takeover of Sicily from the Islamic rulers. The Islamic Abbadid dynasty ruling in Spain falls when the Almoravids storm Seville. Faced with this new threat, the king of Badajoz, al-Mutawakkil ibn al-Aftas, got the support of Castile in exchange for the Muslim positions on the Tagus river — Sintra, Santarém and Lisbon. Roger Guiscard takes Malta.
By topic
Religion
Athanasius VI bar Khamoro becomes Syriac Orthodox Patriarch of Antioch.