Springs

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.
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.