The pyramids are chiefly impressive for their enormous size and the staggering manpower that must have been employed in their construction. The largest is the Great Pyramid of Giza which remained the tallest structure in the world for 3800 years (see List of tallest freestanding structures in the world). The engineering problems involved were chiefly to do with the transport of blocks, sometimes over long distances, their movement into location and exact alignment. It is now generally agreed that the skilled building workers were respected and well treated, but undoubtedly very large numbers of labourers were necessary to provide the brute force.