The round towers vary in height, those remaining perfect or nearly so being from seventy to two hundred feet, and from eighty to thirty feet in diameter at the base. The entrance is twelve to eighteen feet from the ground, the tower being divided into stories about ten feet high, each story lighted by a single window, the highest compartment having invariably four lancet windows opening to the cardinal points of the compass. The roof is conical, made of overlapping stone slabs, and a circle of grotesquely carved heads and zigzag ornamentation is found beneath the projecting cornice. The masonry is of hewn stone, but not the least regularity is observable in the size or shape of the blocks, some being very large, others small, and every figure known to the geometrician can be found in the stones of a single tower. All towers still standing occupy sites noted as historical, and evidence, sufficient to warrant the belief, can be adduced to show that almost every historic spot on Irish soil once boasted one or more of these interesting structures. The existing towers are generally found close by the ruins of churches, abbeys, or other ecclesiastical buildings, and the effect on the landscape of the masses of ruins, surmounted by a single tall shaft, is often picturesque in the extreme. The proximity of the tower to the church is so common as to lead writers on Irish antiquities to conjecture that the former was constructed by the monks who built the church; those advocating the Christian origin of the round tower taking the ground that it was built, either as a place of safe-keeping for valuable property, as a belfry for the church, or for the purpose of providing cells for hermits.