I think a medium/large bell would be a cool construct to have. It could be used as a method of signaling over long distances to sound alarms or otherwise communicate, possibly having a couple different patterns it could be rung in (i.e rapidly for alarm, slower to signal the arrival of someone at the gate). It would be a suitable size to mount on the top of a castle tower and would be made in two stages: The bell (requiring forging) and the stand (involving a stand built with several boards, nails, and a rope).
Rough example: