Four-sided spirit fortress keel diagram

Quick answer：

The 'Four-sided spirit fortress keel diagram' refers to a technical drawing or architectural plan that illustrates the structural framework of a four-sided fortress, focusing on the keel or foundation. In ancient Chinese architecture, these diagrams would be used to ensure the stability and structural integrity of fortresses. The keel, or keel diagram, is crucial as it provides a blueprint for the foundational support of the structure.