How do you Properly Query Posts in a Multisite Network?

Howdy, a brief multisite question for the brain trust:

In a multisite environment, what’s the procedure for querying the posts of a site within a network (as opposed to a traditional config)?

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I’ve got a travelogue in a subdomain that I’m interested in folding into an MS install on the root.

Off the root I’m going to be creating specific country & city Pages, with lists of travelogue entries (relevant to each Page’s city or country).

Simply put, the Page domain.com/peru/lima/ would contain (amongst many other things) a list of all (not just recent) travelogue.domain.com entries from — yep, you guessed it — Lima, Peru.

‘Peru’ is a category and ‘Lima’ is the value of a custom field for that post.

I will ultimately need to query, at the root level, posts in the travelogue that are within a certain category and contain a specific custom field. :p