Shell keep

This is what such structures are sometimes called in specialized literature. The meaning of the term can be stated as follows: initially, a motte or artificial hill is poured in a pre-selected place, enclosed by a stone wall, inside which a garrison is placed. Next, various rooms are attached to the wall from the inside, towers are built if necessary, and the result is a gloomy fortification structure like the one shown in the picture.
Main building of the castle by winter evening.