This essay contains the advice or opinions of one or more Wikipedia contributors. Essays are notWikipedia policies or guidelines. Some essays represent widespread norms; others only represent minority viewpoints.
On many wikis, Wikipedia included, a walled garden is a set of pages or articles that link to each other, but do not have any links to or from anything outside the group. This can be a failure of linkage, or it can be an attempt to form a group of articles on essentially the same topic. This should especially be avoided on Wikipedia, where one of our core principles is building the web. While orphaned articles are far easier to detect than walled gardens, if you find several pages that only seem to link to each other, then be bold and, depending on the circumstances, add both outgoing and incoming links, or suggest a merge.