The bell oven is a large clay structure with an open top. A fire or embers in the bottom of the vessel warm up the entire oven. Bread dough can be pushed onto the hot inner walls of the oven to bake in the heat of the fire.
Indian Tandoor ovens work in this way. All of the best nan bread is cooked in these types of oven. Ancient ovens would have been solid fuelled, the modern counterparts are usually electric or gas powered.
There are a number of commercial suppliers of modern Tandoor ovens,
The Clay Oven Company