Great Ardra, also known by numerous variant spellings, was a West African kingdom formerly located on the Slave Coast in what is now southern Benin. It was named for its capital, the modern Allada, which was also the main city and major port of the realm.
The city and kingdom were supposedly founded by a group of Aja migrants in the 12th or 13th century. Its kings "ruled with the consent of the elders of the people". The state reached the peak of its power in the 16th and early 17th centuries, when it was an important source of slaves for the Atlantic trade. In the mid-16th century, it had a population of about 30,000 people.
According to the Fon oral tradition, the Aja settlers that established themselves in the area of present-day Allada arrived in southern Benin around the 12th or 13th centuries coming from Tado, on the Mono River. They established themselves in the area that currently corresponds to southern Benin, until circa 1600, when three brothers - Kokpon, Do-Aklin, and Te-Agdanlin - split the rule of the region amongst themselves: Kokpon took the capital city of Great Ardra, reigning over the Allada Kingdom, while his brother Do-Aklin founded Abomey (which would become capital of the Kingdom of Dahomey) and their brother Te-Agdanlin founded Little Ardra, also known as Ajatche, later called Porto Novo (literally, "New Port") by Portuguese traders (which is the current capital city of Benin).
Founded by Aja settlers, the settlement of Allada was in 1600 the most prominent of Aja states, bordering the nearby Oyo kingdom, to which the King of Allada was vassal and tributary. Although it was an inland kingdom, Allada maintained control of some sea ports such as Offra, Jaquin and Whydah, thus making Allada important in the growing slave trade business, which also granted Allada the economic means to pay its duties to Oyo.
Originally a part of the Allada Kingdom, the city of Abomey went on to become capital of a new kingdom, the Kingdom of Dahomey, which grew strong enough to challenge the nearby Oyo Kingdom, with Dahomey finally vanquisinhing it and establishing itself as the main Kingdom in the region.
In 1724, the Kingdom of Dahomey invaded the Kingdom of Allada - in three days, the King of Dahomey's troops slaughtered thousands of Allada's warriors and citizens, and made more than 8000 prisoners out of Allada's population, that were sold as slaves to go to the New World" 
Slaves used to be captured from enemy tribes and passed on to European slavers bound for the Americas, the route which by repute the father of Toussaint L'Ouverture, the famous general that started the Haitian revolution that lead Jean-Jacques Dessaline to liberate Haiti, had taken.
Connection to Toussaint L'Ouverture
According to most sources, Toussaint Louverture, the Haitian revolutionary and first Black governor of Haiti then named Saint-Domingue, was the grandson of Gaou Guinou, he was born in 1743, at haut du cap which is a region in today Cap-Haitien. He was the grandson of Gaou-Guinou, himself the heir - either son  or brother  - of the King of Allada who had been killed during the invasion by the Kingdom of Dahomey, in 1724.
According to such sources, "Gaou Guinou, Minister of War and younger brother of the King of Allada, rather than succeeding to his father and ascending naturally to the throne, chose then to accompany his vanquished soldiers in exile. He was given a hammock on board of a slave ship"  that sailed to the American island of Hispaniola, where the slaves were sold in Haiti.
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