Toulouse was integrated into the Roman empire during the 2nd century BC and boasted aqueducts, theatres, therms and a forum. Actually, it was the fourth largest city in the western half of the empire!
Toulouse changed hands quite few times. It became the capital of the Visigothic kingdom, which stretched from the Loire Valley to the Gibraltar Strait, in AD 418; part of the Merovingian kingdom 90 years later (thanks to Clovis I); the chief town of the Carolingian kingdom of Aquitaine after successfully withstanding a Saracens siege in 721; and the seat of the feudal countship of Toulouse after 778.
The early history of the city is marred by the martyrdom of its first Bishop, St Saturnin(us) (or St Sernin).
"[T]o reach the Christian church Saturninus had to pass before the capitol [the Capitole], where there was an altar, and according to the Acts [of Saturninus] were employed as historical sources by the chronicler Gregory of Tours, the pagan priests ascribed the silence of their oracles to the frequent presence of Saturninus. One day they seized him and on his unshakeable refusal to sacrifice to the images, they condemned him to be tied by the feet to a bull which dragged him about the town until the rope broke. (Tellingly, the identical fate was ascribed to his pupil Saint Fermin whose site of martyrdom is at Pamplona.)
The bull, it is said, finished at the place since named Matabiau -- that is, matar ("the killing") and biau or bœuf ("bull"). [...] Two Christian women piously gathered up the remains and buried them in a "deep ditch", that they might not be profaned by the pagans. [...]
The site, said to be "where the bull stopped" is on rue du Taur ("Street of the Bull")." (Source: Wikipedia)
In 403, the site of the Bishop’s grave was used as the building location for Saint-Sernin Basilica.
Toulouse played an important role during the Albigensian Crusade. Raymond VI, its count, was a Catholic and a Cathar sympathizer.
The city was besieged several times. The last assault (1217-1218) saw the death of their leader, Simon IV de Montfort. "Simon stopped to aid his brother Guy, who had been wounded by a crossbow, and was hit on the head by a stone from one of the defenders' siege engines (either the trebuchet or a mangonel), apparently operated by donas e tozas e mulhers (ladies, girls, and women). It killed him. The leadership of the Crusade fell to his son Amaury but the siege was soon lifted." (Source: Wikipedia)
The Dominican order was also established in Toulouse during that time. (Founder Saint Dominic's home was at 7 Place du Parlement, which is less than 200 metres from I used to live. There is a memorial plaque on the building.)
The Inquisition settled in the city in the 1230s, via the Council of Toulouse. Its hunt for heretics contributed to crushing the Cathar movement in the 14th century.
Toulouse had its first university in 1229. Its aim was to teach theology and Aristotelian philosophy, and combat heresy. It is also one of the oldest in Europe.