sentences:  Despite having been left incomplete during the medieval period, Cologne Cathedral eventually became unified as "a masterpiece of exceptional intrinsic value" and "a powerful testimony to the strength and persistence of Christian belief in medieval and modern Europe".When construction began on the present Cologne Cathedral in 1248 with foundation stone, the site had already been occupied by several previous structures.The earliest may have been for grain storage and possibly was succeeded by a Roman temple built by Mercurius Augustus.