ABSTRACT Cities are composed of a varied array of interstices that are often observed as a network of undeveloped, vacant, or leftover spaces and infrastructures. Such conditions, however, little speak on the multifaceted nature of interstitial spaces as in-between domains that can be morphologically heterogeneous, functionally active, and socially constructed; something that expands on the multiple ontologies of interstitiality beyond being vacant, empty, or inert. While acknowledged around their development potentials or as elements of segregation, the physicality of interstices obscures their evolving spatiality between conflict and divide, relationality, and acceptance. This spatiality becomes plausible in post-conflict cities—such as Belfast—where interstitiality takes the form of “interfaces”: complex in-between areas that articulate divided Catholic/Nationalist and Protestant/Unionist communities throughout peacebuilding politics. By taking a phenomenological approach, the spatiality and meanings of Belfast interfaces are scrutinised to find that Belfast interfaces speak on multiple ontologies linked to (i) their evolving spatiality, (ii) their constructed meanings, and (iii) their naturalisation as daily-life transitions that draw on emerging forms of safety, identity, memory, integration, segregation and futurity. Belfast interfaces take the ontological turn of the in-betweenness towards suggesting that the staging of spaces as promoting togetherness can be more illusionary than concrete.