Their vaults were heavily constructed at no great height from the ground, and generally applied to the aisles of churches. They exerted a greater thrust on the walls than the later Gothic vaults. _Norman_.--These churches are generally to be found perched on commanding sites, chosen as natural places of defence. Often a river wound round the base, and where it led short, a moat was constructed on the landward side, and borrowed its water from the river. The activity of the Norman builders is astounding, and forms a great contrast to the few years before their advent. For a short time architecture suffered a paralysis. Not till the much-dreaded Millennium (1000 A.D.), when it was thought the world would certainly come to an end, had passed did people take heart again, and architects make up for lost time. _Early English_.--In this period the massive Norman walls gave way to walls reduced in thickness. The buttresses became of more structural significance. Also, flying-buttresses gradually came into use to strengthen the weakness of the upper works, caused by the reduction of the walls in thickness. The pillars were elongated, and of slight construction. The doorways, windows and arcades were built with polished marble obtained from the Isle of Purbeck. The science of vaulting became more advanced. The towers were taller and more elegant, with plain parapets. They were generally furnished with windows. The lower ones resembled much the arrow-slit formation of the Norman style. The upper windows were grouped in twos and threes. The broach-spire now came into notice. It was added on to the square tower, and at the early part of this style was low in height, but gradually became taller. The circular-headed windows of the Normans gave place to the narrow-pointed lancets of the Early English. These admitted little light, and necessitated a greater number of windows, which were grouped into couplets or triplets. _Geometrical_.--The window, by the gradual process of piercing the vacant spaces in the window-head, carrying mouldings around the tracery (or ornamental filling-in), and adding cusps (the point where foliations of tracery intersect), gave rise to Geometrical work. The earliest work of this kind is found in Westminster Abbey. _Decorated_.--The towers are made to appear lighter by the parapets being either embattled or pierced with elegant designs, and pinnacles placed on them. The broach-spires gave place to spires springing at once from the octagon. The buttresses are set angularly. In this period the architects failed to maintain the vigour of the Geometrical period. The Decorated windows are formed of portions of circles, with their centres falling on the intersection of certain geometrical figures. There is a glorious example afforded by the west window at York.