Though as an inflected language, Latin can be freer with word order than English, especially in poetic works, it is really unusual to so separate the preposition *in *and its object virtute tua. The *Vulgata Clementina *has the more reasonable Fiat pax in virtute tua.
I would have said “Let peace be in your strength”, but since the psalm is speaking to Jerusalem, I suppose “walls” is a poetically licensed translation. (However, I suspect the “wall” interpretation is more from the Hebrew – The *Nova Vulgata *has *Fiat pax in
*, which is definitely “your walls”)