In 15 3 7 the island, then a prosperous Venetian colony, was overrun and ruined by the pirate Barbarossa (Khair-ed-Din).
The emperor Frederick Barbarossa was the last to wear the insignia (in 1167).
Sali Reis, also by birth a Christian of Asia Minor, was likewise successful as a corsair; he distinguished himself especially at the capture of Tunis, and succeeded Hassan Barbarossa as beylerbey of Algiers.
Alessandria was founded in 1168 by the inhabitants of the district in order to defend themselves against the marquis of Monferrato and the town of Pavia, at whose request it was besieged in 1174 by Frederick Barbarossa for six months, but without success.
In 1189 Frederick Barbarossa of Germany sought and obtained leave to lead his troops on the third crusade through the Byzantine territory; but he had no sooner crossed the border than Isaac, who had meanwhile sought an alliance with Saladin, threw every impediment in his way, and was only compelled by force of arms to fulfil his engagements.
On this headland stands an abandoned monastery of St Anthony, amidst the ruins of a medieval castle, which belonged to the Venetian family of the Venieri, and was gallantly though fruitlessly defended against the Turkish general Barbarossa in 1537.
In 1157 an almost circular moat, still preserved in the inner canal or Naviglio, was constructed round the town; but in 1162 Frederick Barbarossa took and almost entirely destroyed the city, only a few churches surviving.
Refused to truckle to the municipality, placed it under an interdict (1155), and allied himself with Frederick Barbarossa to quell an insurrection which respected the rights of emperors no more than the rights of popes.
For at the beginning of his reign Valdemar leaned largely upon the Germans and even went the length, against the advice of Absalon, of acknowledging the overlordship of the Emperor Frederick Barbarossa at the reichstag of Dole, 1162.1162.
In 1865 appeared his monograph on Henry the Lion, duke of Saxony and Bavaria, which was followed by three volumes on the emperor Frederick Barbarossa (Kaiser Friedrich I., Danzig, 1871-1874).