When townhouses are built in a row, the roof structure frequently runs across all of them as one continuous void. Party walls sometimes stop at ceiling height rather than continuing up to the underside of the roof, which means a person in the ceiling space above one unit is, in structural terms, above several. Nothing about that is visible from inside any of the homes involved.
It is worth checking rather than assuming, because developments vary and many are built correctly with the party wall carried right up. The way to find out is to look: a manhole, a torch, and whether you can see continuous space running past your own boundary. If you can, that is worth knowing about, and it is a construction matter as much as a security one.
The rest of the work here is the ordinary business of complex living — unit turnover, communal gates, entrance hardware and the shared-door questions that any scheme produces. That is lock change services and body corporate work, and it makes up most of what we do in the suburb.