Am I allowed to sub-let my room or apartment?

No, it is not possible to sub-let a Citylife room or apartment. However, if a subset of members wish to move out of their apartment while another subset wishes to remain, it's possible to replace the leaving members' names in the contract with the names of those who will replace them. 

Members are responsible for finding their successor members themselves, and will be charged an administration fee of €150 for the drafting of a new tenancy agreement.