There are various synonyms that can be used in place of the term "walled-in" depending on the context. For instance, if you want to describe something that is securely enclosed, you could use the term "enclosed" or "fenced-in." If you wish to express the notion of a place being isolated, "segregated" or "quarantined" could be your choices. Meanwhile, "restricted" or "confined" could be used if you wish to imply the sense of limitation or confinement. If you want to express the idea of being enclosed by walls, then "fortified" or "encased" could be the ideal words to use. Ultimately, the synonym you use will depend on the context in which you intend to use it.