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:Celje Castle was once the largest fortification on Slovenian territory. The first fortified building on the site (a Romanesque palace) was built in the first half of the 13th century by the Counts of Heunburg from Carinthia on the stony outcrop on the western side of the ridge where the castle stands. It had five sides, or four plus the southern side, which was a natural defence. The first written records of the castle date back to between 1125 and 1137; it was probably built by Count Gunter. In the western section of the castle, there was a building with several floors. Remains of the walls of this palatium have survived. In the eastern section, there was an enclosed courtyard with large water reservoirs. The eastern wall, which protects the castle from its most exposed side, was around three metres thicker than the rest of the curtain wall. The wall was topped with a parapet and protected walkway.