To purify the water from a certain river the monks have carved a section of the river that goes over a stone bed. Through holy carvings of gods, godesses, and holy symbols and then the river flowing over them, it purifies the water making it safe for drinking and/or makes it holy water. For rivers that do not have natural rock bottoms, large smooth stones can be carved and added to the water to fully cover the bottom.
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.
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.
The city of Nausopol is built on stilts. Lots of very sturdy stilts and butresses, of course, because it rises about five hundred feet from the ocean. Even the most terrific of storms is only heard in the city as a distant cacophony of blasts as waves strike the solid stonework fathoms below. It has never been attacked because of its isolation and impregnability.
It's not a place for the faint-hearted: vertigo and sea-sickness are not desirable traits. But when you are standing in the middle of the city there is no way you could tell that you were standing above an ocean, separated only by a gulf of air and a few stones.
A thousand steps lead down from Nausopol to the floating docks. These docks are pitch-coated wooden and can be raised by winches during squalls. Trade with other cities and countries is good: Nausopol is built over a sunken atoll whose minerals are still mined by divers, and it was from this that it originally derived its wealth.
But the principal method of getting to and from the city is by riding the giant sea-eagles which have been captured and bred for that very reason.