So one way we can do this (assuming I've understood correctly) is...
Create a set off [City]; and make it conditional and use this formula
MAX(IIF([Event Number]=[Enter Event Number],1,0))=1
Bring this onto the filter shelf and now a selection of an Event will filter to the City of that event. This works a bit like a FIXED LoD.
Hope that is what you were after and makes sense.
Create Set on Lower Level.twbx 19.4 KB
Worked like a charm, thanks Simon!