ISAC is emerging as a foundational capability in 6G, enabling mobile networks to not only offer communication services but also to sense and perceive their environment at scale. This paper explores architectural considerations to enable sensing in 6G, extending on recent developments by (pre-)standardisation bodies such as 3GPP and ETSI. Selected ISAC use cases are presented from the European MultiX project including associated potential functional system requirements. The paper proposes a 6G system architecture that integrates newly proposed NFs for the purpose of sensing and demonstrates how they are being used in offering sensing as a service. Protocol stack adaptations for both control and a newly proposed sensing plane are discussed.