The detailed 3D documentation of mediaeval castles in heavily accessible topographic settings or partly hidden behind dense vegetation encounters major challenges to conventional topographic, geodetic or photogrammetric surveying. This paper presents a rapid solution for the detailed documentation of the upstanding architecture combining terrestrial and unmanned laser scanning (TLS/ULS) and motorized high resolution ground penetrating radar (GPR) for the detection and documentation of remains hidden in the subsurface. The large mediaeval castle Rosenburg, Lower Austria situated on a prominent mountain ridge provided a typical situation for a respective case study to present a state-of-the-art solution for quick and efficient surveying above and below the surface. The used terrestrial laser scanner RIEGL VZ-400i is able to perform hundreds of scans in one day, which can then be registered automatically. An RIEGL VUX-1UAV laser scanner system mounted on the multi-copter RiCOPTER was applied to measure the roof landscape in minutes and supplement the ground-based data acquisition. A motorized multi-antenna GPR system MIRA measures delivers additional information on subsurface remains like vaults, cisterns, walls of previous buildings up to the remains of garden layouts. We present the latest developments in data acquisition, processing and fusion of such 3D data sets for the documentation of historical buildings or archaeological sites with still standing architecture including the fast and efficient production of 2D maps and plans from the dense 3D data sets. Relevance for the conference: state-of-the art combination of TLS, ALS and GPR Relevance for the session: Digital documentation of historical buildings and archaeological sites Innovation: Fusion of laser scannig and ground penetrating radar data