Generate a futuristic eco-friendly architectural structure nestled within a lush, misty mountain valley. The structure is a multi-tiered, circular building with concentric layers, each adorned with greenery and rooftop gardens, reflecting sustainable design. The building's smooth, metallic surfaces blend with natural elements, and large, panoramic windows are visible around each tier, glowing faintly to imply occupancy. Surrounding the structure, towering, sheer cliffs covered in dense green foliage rise on either side, partially obscured by low-lying clouds and mist. The atmosphere is tranquil and ethereal, with soft, diffused lighting filtering through the clouds, casting a cool, natural glow over the entire scene. The setting is reminiscent of a hidden, advanced civilization harmonizing with nature.