Armor.

Very slowly, with medieval (hand) tools.

In the 13th century soldiers began to strengthen their protective mail. They used metal plates to protect the places on their bodies that were exposed despite the mail chains.

A bullet proof jacket is made from several thin layers of a material known as kevlar ( Kevlar is a registered trade mark). Kevlar is a para-aramid synthetic fibre. Some vests also allow for metal plates to be inserted. While kevlar is light-weight, inserted metal plates make the vest heavier. Though if the vest stops a bullet, I doubt the wearer would mind how heavy the vest was.