Mosteiro de Santa Clara-a-Velha, Coimbra.
The medieval Monastery of Santa Clara-a-Velha in Coimbra has been abandoned since 1677. The ground floor was constantly flooded with water and for centuries all kinds of works to raise its floor were carried out. However, the problem just got worse and worse, and up to a few years ago you could still visit the old ruin in semi-clandestine fashion and witness what looked like the fanciest gothic cistern you ever saw. Alas, EU money, the obsession with sanitizing everything and will to spend public money now have made it an open monument, with no water… constantly being drained by motorized pumps. A real shame and waste of resources (in our modest, purist opinion) but what you can see is still great.
Like these steps leading to a hole in a wall. By way of a small gap you could communicate with the church inside, and so the nuns could confess their many sins without mingling with the crowd.