grayscale heightmap 3d model

Generate a grayscale heightmap image for a 3D tower defense game map. Requirements: - Large flat area in the CENTER for player base and factory buildings - High mountains surrounding the edges (white = high, black = low) - 2 to 3 narrow valley passages through the mountains for enemy entry paths - Some small hills and rocks scattered inside the flat center area as obstacles - Smooth gradients between heights - Pure grayscale, no colors - Resolution: 1024x1024 - Top-down view heightmap style