These houses were built either of adobe or of stone, plastered over with adobe mud; and nearly all those on the ground floor were entered, as Robinson Crusoe entered his castle, by climbing a ladder to the roof, and descending another that led down through a skylight. Thus, if an enemy should succeed in forcing his way through the narrow tunnel into the plaza, the people would merely retire to their house-tops, draw up their ladders, and he would find it as hard to get at them as ever.