  By interacting, synchronizing, and cooperating with its physical counterpart
in real time, digital twin is promised to promote an intelligent, predictive,
and optimized modern city. Via interconnecting massive physical entities and
their virtual twins with inter-twin and intra-twin communications, the Internet
of digital twins (IoDT) enables free data exchange, dynamic mission
cooperation, and efficient information aggregation for composite insights
across vast physical/virtual entities. However, as IoDT incorporates various
cutting-edge technologies to spawn the new ecology, severe known/unknown
security flaws and privacy invasions of IoDT hinders its wide deployment.
Besides, the intrinsic characteristics of IoDT such as \emph{decentralized
structure}, \emph{information-centric routing} and \emph{semantic
communications} entail critical challenges for security service provisioning in
IoDT. To this end, this paper presents an in-depth review of the IoDT with
respect to system architecture, enabling technologies, and security/privacy
issues. Specifically, we first explore a novel distributed IoDT architecture
with cyber-physical interactions and discuss its key characteristics and
communication modes. Afterward, we investigate the taxonomy of security and
privacy threats in IoDT, discuss the key research challenges, and review the
state-of-the-art defense approaches. Finally, we point out the new trends and
open research directions related to IoDT.
