That this great circumvallation, 983 m.--3,225 ft.--in circuit, was a wall for protection also there is no doubt, although the main strength of the pueblo lay in the construction of its houses, where the inhabitants could simply shut themselves in and await quietly until the enemy was tired of prowling around it. By Indians it could only be carried by surprise or treachery.[187] Hence it was customary for the young men to leave the pueblo at times in a body, abandoning it to the old men and women, etc., without concern.[188] As long as these kept good watch they were safe, even if the Comanches should appear. Roaming Indians cannot break open a pueblo house if well guarded. For that purpose alone the mounds near the great gate, and the mound _H_, Pl. IV., were erected. They were watch-towers for special purposes, for particular sections, where the lookouts from the wall-tops were not sufficient.[189] These two mounds--one on each side of the gateway--overlooked the fields and the creek-bank: in the morning, when the people went out to work, or to carry drinking water from the spring opposite; during the day, while they attended to their simple labor of tillage. The mound and tower _H_ performed a similar office towards the steep ledge of rocks there descending, among whose fragments Indians could hide for hours from the scouts on the house tops. Thus the great enclosure with its details served a triple purpose. It was the reservoir which held and conducted the waters precipitated on the _mesilla_ to the useful purpose of irrigation. It was a preliminary defensive line,--a first obstruction to a storming foe, and a shelter for its defenders. But it was also in places an admirable post of observation. It formed the necessary complement to the houses themselves,[190] and both together composed a system of defences which, inadequate against the military science of civilization, was still wonderfully adapted for protection against the stealthy, lurking approach, the impetuous but "short-winded" dash, of Indian warfare.