The preservation of historical built heritage with low accessibility involves specific challenges throughout preliminary knowledge acquisition and diagnosis, due to operational difficulties in collecting and complementing heterogeneous data from direct survey, diagnostic investigation and monitoring. In this context, digital solutions, also usable in Virtual Reality (VR) and Augmented Reality (AR) environments, are catalysts for a new vision that enhances understanding of the geometric, dimensional, and material characteristics, while facilitating semantic enrichment with informative contents and products and real-time retrieval of data from onsite monitoring sensors. In this framework, the paper presents the two-year project “DIGIT-ACCESS. DIGITal gateway for low ACCESSible heritage architectures”, recently funded by the Italian Ministry of University and Research—Grant Program PRIN 2022 PNRR (Projects of Relevant National Interest). The project aims at testing and validating a set of operational protocols and digital solutions for the exploitation of image-based models and environments as replicas of architectural sites to be accessed remotely, hubs for visualization and querying of experimental data from non-destructive investigations, sensor networks, and analytical simulations, as well as databases for the application of image-processing routines and machine-learning methods toward identification of degradation patterns and diagnosis of the state of conservation. The proposed outcome is a DIGITal gateway to support assessment and management of low ACCESSible architectures, to be validated in representative case studies of underground sites in Southern Italy.