Flat Design Spirit Fortess

Quick answer：

Flat design, also known as 'flatter design', is a design style that emphasizes simplicity and minimalism. It typically involves the use of flat colors, simple shapes, and lacks textures or shadows. The 'Spirit Fortess' in this term seems to refer to a concept or element within the field of graphic design that embodies strength, resilience, or a guiding principle, akin to a metaphorical fortess that represents the essence of flat design.