Fortress refers to frontier fortress and fortress. The so-called fortress here refers to the Great Wall of past dynasties, and the northern part has left the frontier fortress, so it is named "Northern Fortress" and is occasionally used with the Great Wall.
The word Saibei, as a geographical division, is not a simple concept of orientation like northeast and southwest. Plug means border, and Saibei means beyond the border. I vaguely think that these places are not the territory of China, but in fact all the provinces and regions in Saibei are the territory of China, so today's Saibei and the Great Wall should be regarded as Sene.
After liberation, due to the independence of Outer Mongolia, all the other four provinces were abolished after the administrative division was reset (the name of the province was only "Ningxia", but the number of provinces and regions was greatly reduced), and some areas were included in Hebei Province and Ningxia Hui Autonomous Region, all of which became Inner Mongolia Autonomous Region. Because there is only one administrative region left, and Inner Mongolia itself includes not only the northern part of Saibei, but also the western part of Northeast China, and for the above reasons, it is geographically classified as North China, that is to say, it is the northern part of China territory, not the area north of China territory.
After the government of the Republic of China moved to Taiwan, the zoning methods and concepts of Saibei continued to be used in geography textbooks. It was not until the educational reform began in the 1990s that textbooks were opened. In order to conform to the realistic division, most of them were no longer used, but the geographical division was different from that of the mainland. Instead, Inner Mongolia is divided into the northwest and shared with Xinjiang, while the local Mongolia (that is, outer Mongolia) is moved to world geography.