Metal is a material which has been used in every aspect of architecture - from providing waterproof covering to holding the doors onto the frame.
Even though it is largely the Georgians and Victorians which initiated the apotheosis of the use of metal in buildings, it has been used for structural and decorative purposes for thousands of years. It is known , for example, that the Roman Pantheon had a bronze roof.
For decoration there is nothing like metal to bring detail into relief (see the photo of a door at Selby Abbey above)
Key moments in the use of metal as a structural form relate to its first use to construct a bridge at Ironbridge in Shropshire by Abraham Darby in 1779 and the building of the Crystal Palace by Joseph Paxton in 1851
To see a comprehensive set of images showing the use of metal in architecture see here