As we move closer to the 6G era, the complexity and dynamism of communication networks is set to increase significantly. This requires more sophisticated, automated management solutions, while retaining human oversight. To meet these new challenges, it is essential to build new architectures that provide a secure environment for closed-loop network automation and to enable optimal resource planning, management and control. In this context, this paper presents the vision of the European 6G-TWIN consortium, which focuses on the creation of a cyber-physical continuum that seamlessly merges the physical world with its digital representation, through the concept of Network Digital Twin (NDT) and its integration into an Artificial Intelligence (AI)-based 6G architecture. This initiative aims to bridge the gap between the increasing complexity of networks and their operational performance, potentially improving real-time user experiences across multiple domains. This paper specifically highlights the associated key challenges and the approach taken by 6G-TWIN to address them.