Usually compost bins are made of plastic, iron or wood. And they have at least one side that can be opened. But there is nothing wrong with building a large compost bin from stones.
However, it is advisable to leave one side open or to install a door or a flap. After one or two years, the mature compost will be in the lower third of the compost pile. You should be able to get to the valuable humus directly without digging up the entire pile. Very important: The soil must remain open at the bottom. So do not lay stone slabs or pour everything in concrete! Animals, such as earthworms, get into the compost pile from below. Only through them can the decomposition process really take place.