Zen garden design is a traditional Japanese art form that aims to create a tranquil and harmonious space. The sunken courtyard, also known as a 'teikakuden,' is a key element in this design. It involves lowering the garden level to create a sense of seclusion and to focus on the garden's natural elements such as stones, water features, and plants. The design emphasizes simplicity, symmetry, and the naturalistic representation of nature's beauty.