Your example isn't that big though, not compared to a castle. Doors were sometimes made of metal. Portcullis

From: https://en.wikipedia.org/wiki/Naval_ram >The Athlit ram consists of a single bronze casting weighing 465 kilograms (1,025 lb). It is 226 centimetres (89 in) long with a maximum width of 76 centimetres (30 in) and a maximum height of 96 centimetres (38 in).  ... The casting of an object as large as the Athlit ram was a complicated operation at the time, and would have been a considerable expense in the construction of a war galley.

The ram was MUCH smaller than castle walls. It was 7' long by 2.5' around. A castle wall could easily be 10' tall and several feet thick. A 10' section of wall would need maybe 6 times that much metal and castle walls are far longer than 10'. Metal walls would get wildly expensive very quickly.