5G is envisioned as a transformation of the communications architecture towards multi-tenant, scalable and flexible infrastructure, which heavily relies on virtualised network functions and programmable networks. In particular, orchestration will advance one step further in blending both compute and data resources, usually dedicated to virtualisation technologies, and network resources into so-called slices. Although 5G security is being developed in current working groups, slice security is seldom addressed. In this work, we propose to integrate security in the slice life cycle, impacting its management and orchestration that relies on the virtualization/softwarisation infrastructure. The proposed security architecture connects the demands specified by the tenants through as-a-service mechanisms with built-in security functions relying on the ability to combine enforcement and monitoring functions within the software-defined network infrastructure. The architecture exhibits desirable properties such as isolating slices down to the hardware resources or monitoring service-level performance