The corners of a medieval fortress were sensitive parts, because easier to attack and hard to defend. And when the enemy reached the top of the wall, he was hidden by pieces of masonry and hard to repulse. So, the "towers of the corner", as they were called, had to be very strong. They were built larger and taller than the other ones. At the bottom of these towers, there were wider ditches, fences, and sometimes advanced forts or bastions. We find also some "tours du coin" in the city of Carcassonne, in the castle of Pierrefonds and the well-known fortress of "la Bastille".