Memory access circuitry (26) enforces ownership rights for memory regions. A given memory region is associated with an owner realm specified from multiple realms, each realm corresponding to a portion of at least one software process executed by processing circuitry (8). The owner realm has a right to exclude other realms from accessing data within the memory region. Realm management circuitry (20) accesses a realm management tree storing realm management data for at least two realms in a tree structure having a variable number of levels. The realms are identified using a realm identifier which has a variable number of variable length bit portions each providing an index into a given level of the realm management tree.