"outer walls" is a correct phrase in written English.
You can use it to refer to the walls that face the outside of a building, as in "The outer walls of the castle were covered in ivy."
The brick outer walls were ten feet high.
The outer walls, made of rammed earth, protected against bandits.
"Only the outer walls were left — it looked like an airplane hangar!" Saban told me.
The headstones stretch to the outer walls; there are no paths between them.
What matters most, of course, is the auditorium, with its lush red outer walls.
Dragons are projected on to the outer walls; flaming torches line the approach.
Five cameras sit on dolly tracks in the space between the inner and outer walls.
Right away I noticed the Indian soldiers carrying rifles standing on the outer walls and rooftops.
He preserved Lapidus embellishments like the perforated "Swiss cheese" outer walls.
Defenders paced the outer walls of the mosque holding guns and sharpened garden tools.
And the apartments were hung on new outer walls to maximize light and their views.
