Arco do Castelo means Arch of the Castle. In spite of its ancient looks, this arch was only built in the mid-19th century.
It is a fine example of the architectural style inspired by Romanticism, which placed its emphasis on emotion, and the glorification of all the past and nature, focusing particularly on the medieval period.
On top of the gateway, in the middle, you will find the ancient coat of arms of the Kingdom of Portugal, with the date written below it.
Now go across the arch and keep going until you find the statue of St George.