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 journey had been a long one and now they had entered the mountains. After an entire day spent on paths cut into the mountainside and through moss covered coniferous woods, they encounter a desperate young mountain tribal. He is searching for his beloved wife and his elder brother, both of whom he got separated from after an unsuccessful attempt to kill a cave bear. The last thing he saw was the cave bear in full pursuit of his wife. He could not help her as he had been knocked to the ground by the raging beast, and was struggling to regain full consciousness. The tribal will be clearly nervous and urges them to look for his wife during their travels. He will stay in his village a couple of miles to the north and pleads for assistance should they recover his wife, whether she be dead or alive.
On the next day of travel, they will journey upon a dim track in the forest and while they are preparing to ascend another path cut into the mountainside, they hear moans of lust from somewhere nearby. Upon closer inspection they will spy a young tribal woman in the heat of the act with an elder tribal male. They are consummating their forbidden love on the cold mountain moss, and beside them lay the skinned and slaughtered carcass of a huge bear.