As of 2024, Autodesk has retired the 2015 version. You cannot download installers for this version directly from their main website without a legacy contract.
Disclaimer: This post is for educational and archival purposes only. Autodesk Civil 3D 2015 is no longer supported by Autodesk. You must own a valid perpetual license or subscription to use this software. We do not condone piracy. download civil 3d 2015 google drive