Built at the beginning of the 2nd century AD, the Selley Theatre could accommodate around 12,000 spectators, making it one of the largest buildings of its kind. It was modified in the Gallo-Roman age. Both the orchestra and the cavea (tiered seating) are more than semicircular, the rather modestly sized stage building was built onto the exterior of the linear façade wall, and the actual stage consists of a simple wooden platform which reaches into theorchestra.
In the latter part of the 3rd century i twas finally turned into a fortified retreat enclosed by a defensive ditch and was used as such until the mid-4th century.
References:The Citadelle of Quebec is an active military installation and official residence of both the Canadian monarch and the Governor General of Canada. It is located atop Cap Diamant, adjoining the Plains of Abraham in Quebec City. The citadel is the oldest military building in Canada, and forms part of the fortifications of Quebec City, which is one of only two cities in North America still surrounded by fortifications, the other being Campeche, Mexico.
The first fortifications in Quebec were built by the Governor General of New France Louis de Buade, and completed just in time for the Battle of Quebec in 1690.
After the British conquest in the second half of the 18th century, the problem of Quebec City's defences grew more acute.