But you've probably never heard of the Primeira Circular.
Yes, it did exist. It is believed to have been established in 1852 and represented the city limits at that time: starting at the Alcântara Bridge and ending at Cruz da Pedra, in Santa Apolónia. In other words, it would include what is now the stretch of CRIL/IC17 between Algés and Benfica and the current Avenida Dr. Alfredo Bensaúde. Along its route, there were gates and customs offices where duties were paid for goods entering the city.
In the 1940s, the Segunda Circular was created as part of the General Plan for Urbanization and Expansion of Lisbon, which established its expansion to the north. In 1960, the section between Campo Grande and Benfica was created, followed by the section between Campo Grande and the Airport, and then up to the city entrance in Prior Velho. In the 1990s, the final stretch was completed in Calhariz de Benfica. The Segunda Circular covers the parishes of Benfica, São Domingos de Benfica, Carnide, Lumiar, Olivais, and Alvalade.