Maritime surveillance is crucial for achieving maritime situational awareness (MSA) and to effectively prevent, protect, and respond to critical security challenges (e.g., illicit commercial activities, irregular migration, pollution, terrorist attacks). CROSSMARE aims to overcome existing challenges in border and maritime surveillance by introducing a layered maritime surveillance network and an interoperability framework that integrates the space, air, surface, and underwater domains within a comprehensive monitoring picture, enhancing MSA and surveillance capabilities. To achieve this, a consortium of 17 big industries, SMEs, RTOs, and end-users a) utilize cutting-edge technologies in the context of maritime surveillance: High Altitude Platform Stations; Remotely Piloted Aircraft Systems; High-Res SAR Sat imagery; Unmanned Aerial Vehicles; Autonomous Underwater & Unmanned Surface Vehicles; novel Flash LiDAR sensors; as well as legacy surveillance systems, b) apply innovative AI algorithms; Large Language and Visual Models; Advanced multi-modal data fusion, c) extend the capabilities of existing C2 and HMIs, also supporting intelligent mission management of manned and unmanned systems, d) and support the interoperability of novel and legacy surveillance systems, improving cross-border and cross-agency collaboration. To this end, CROSSMARE aims to demonstrate interoperable data exchange with CISE and proposes an extension to the current CISE data and service model, enhancing existing and enabling new functionalities. The project will validate the proposed technologies through a series of validation activities: a) continuous integration testing, b) 2 Small Scale Tests (Italy, Lithuania), c) a Table Top Exercise (Romania), and d) a Full Scale Demonstration (Spain) involving real assets, end-users and relevant stakeholders. A Security and Privacy-by-design framework will ensure compliance with fundamental rights, privacy and personal data protection regulations.