A metal grating, sometimes with a wood backing, with locking bars and hinges used to secure and barricade narrow passages against attack. They are still to be found in many churches on the English-Scottish border; these churches have extra protection, e.g. stone-vaulted ceilings which would not burn. The villagers would hide themselves in stone towers during murderous cross-border raids, using a yett in a specially narrowed passage which could be defended easily and was fire-proof. 'Yett' is a northern form of 'gate
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.