As in those lovely, coiled devices that can store energy.
Coiled springs were invented in the 15th century, and different forms of bent metal to store energy existed before that.
I think they would be a nice, simple addition for one thing - automatically closing gates. A spring installed on a gate will close it a few seconds after it's been opened. I'm sure there could be other uses in the game (aside from inspirationals, of course), but this is my primary reasoning.