The Chinese, when attacking a castle or fort, flew kites over the city wall and used the length of string it took to get it there as a measurement to know how far they had to dig a tunnel to get under the wall.

Silk Armor? The Mongols wore silk undergarments under layers of leather armor. Why? Because silk is very strong. If an arrow hit them and made it through their armor, it would usually not have enough energy to puncture the silk. It could still enter their body but, because the silk would not break, the arrow's barbs could not do their work and the arrow could easily be removed leaving a relatively clean wound considering normal arrow wounds.

A new substance is found in an out of the way area and kept very quiet. It is found to make a perfect counterfeit diamond. Only the most expert of expert appraisers would even have a hope of noticing that it is fake. Only one problem: if it gets wet it starts to slime and if it is saturated, it turns into a glob of gel.

The lengthy process to do something necessary or a magical scroll needed is inscribed on the other side of a great tapestry and can't be removed. For more fun it could be in the kings throne room. Try to hide in shadows with a 20*5 (yards/meters) monstrosity!

A tree was given as a gift at one time to a city. Every year the tree is the center of a large parade as the tree is towed by oxen or large horses in a large wagon through town.

Village has a nearby natural substance that comes from the ground and burns well when lit. They use it only locally and try not to let the secret get out.