Medieval curfews, also known as “couvre-feu” or fire covers, are ceramic or iron covers used to keep the embers of the fire overnight. They are an example of how doing things the medieval way is easier than trying to adapt modern ways to this lifestyle.
The current convention, and a requirement of many sites, is to put the fire out each night ,and then restart the fire each morning.
This causes a lot of additional work, particularly if water is used to put the fire out each night, as new kindling and tinder must be used to build up the fire to the ember stage.
Using a curfew to keep embers overnight, allows the initial work to be skipped, and the fire can be built back into a proper cooking fire quite quickly. It also takes less skill and know-how to get a fire going again when there are embers as a starting point, compared to starting completely from scratch.