The Monastery of Serra do Pilar has been a Cultural Heritage of Humanity site since 1996 and provides an unequalled view of the city of Porto. Its construction began in 1538 and it was completed in 1670. It housed the Augustinian Friars of the Monastery of Grijo until it became occupied by the Liberal army during the Civil War of 1832-1834, during which it was damaged. The Royal Brotherhood of Nossa Senhora do Pilar, established in 1834 and, later, the Group of Friends of the Monastery of Serra do Pilar, created in 1925, helped save the monastery.
The church with its circular floor plan is unique in Portugal, as is the Cloister, which is also circular and is lined with 36 Ionic columns. It currently houses North Heritage. In the cloister, note the statue of D. Afonso Henriques, by Soares dos Reis, which is the plaster version of the one in bronze at the Guimarães Castle. It is worth taking the guided tour which includes a climb to the dome. It's a 100 steps to the circular terrace that surrounds the dome of the church, but the panoramic view over Porto is spectacular.
Serro de Pilar is part of the UNESCO World Heritage site Historic Centre of Oporto, Luiz I Bridge and Monastery of Serra do Pilar.
References:The ancient Argos Theater was built in 320 BC. and is located in Argos, Greece against Larissa Hill. Nearby from this site is Agora, Roman Odeon, and the Baths of Argos. The theater is one of the largest architectural developments in Greece and was renovated in ca 120 AD.
The Hellenistic theater at Argos is cut into the hillside of the Larisa, with 90 steps up a steep incline, forming a narrow rectilinear cavea. Among the largest theaters in Greece, it held about 20,000 spectators and is divided by two landings into three horizontal sections. Staircases further divide the cavea into four cunei, corresponding to the tribes of Argos A high wall was erected to prevent unauthorized access into the theatron and may have helped the acoustics, but it is said the sound quality is still very good today.
Around 120 CE, both theaters were renovated in the Roman style.