The 3D model of the Old Mansion would be a highly detailed and realistic representation of the majestic estate. Starting with the exterior, the Gothic architecture would be prominently showcased, with intricate stonework and ornate carvings meticulously designed. The aged stone walls would display signs of weathering and moss, adding to the authenticity and historical charm.

The entrance gate would feature ornamental ironwork, while the surrounding environment would include the lush grounds with overgrown grass, trees, and creeping ivy that embrace the building. The atmosphere would be set by adding appropriate lighting and shadows, giving the scene an ethereal and mysterious vibe.

Upon entering the mansion, the grand foyer would be the centerpiece of the interior. It would exhibit the double-height ceiling, adorned with a detailed chandelier that casts realistic lighting effects throughout the space. The marbled floors would showcase intricate patterns, and the walls would be enriched with finely crafted crown moldings and dark wood paneling, showcasing its historical significance.

The grand staircase would be a stunning feature, complete with intricate balustrades and a red velvet runner flowing gracefully down its steps. The upper floors would hold various rooms and hallways, each distinct in its design and decor. From the parlors to the library, the 3D model would exhibit fine antique furniture, plush textiles, and captivating artwork on the walls.

In the dining room, the long oak table would be adorned with detailed cutlery, glassware, and table settings, making it seem ready to host a sumptuous banquet. The ballroom would feature polished wooden floors, reflecting the soft glow of the shimmering chandeliers above, as if preparing for a grand dance.

As you explore the mansion's corridors, you might discover hidden doors, secret passages, and other intriguing details, creating an interactive and immersive experience within the model.

Outside, the 3D model of the garden would showcase a vast array of vegetation, including colorful wildflowers, ancient trees, and a meandering cobblestone pathway leading to various garden features like fountains, statues, and a charming gazebo.

In sum, the 3D model of the Old Mansion would be a captivating and immersive experience, capturing the timeless charm, elegance, and historical significance of this storied estate.