Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Ancient Ruins Free.
Ancient Ruins v1.1
Ruins combined with 481 objects, all with background have 25,259 polygons or 24,928 verts.
Textures in TIF file format, 512×512
You can use the ruins as-is, or you can easily rearrange objects for your own needs.