There is one here: Mirilla - a spy-hole - at 66 Rúa do Vilar. Another at 44 Rúa do Vilar.
They had the same idea in Venice.
It's a poor translation. The 'trapdoors' are not in the roofs of the houses, but in the floors of the rooms above the arcades. I'd like to know where there is another in Rúa Nova.
"These peculiar 'hatches', many of which have been eliminated, were used by the tenants of the homes to observe and listen to what was happening on the street without being seen." The people looking down would not see what was happening on the street. I believe the main function of the hatch was to see who was knocking at your door. When I was looking into the feature, I found they were called 'mirillas', which is translated by Google as 'peepholes'. 'Trapdoor' seems misleading.