The phrase "mountainous backdrop" means that there are large hills or mountains in the background of something, like a view, photograph, or setting. Full definition
The unique landscape features beautiful beaches set against mountainous backdrops. (luxuryretreats.com)
The owner Franz Hubbel, whipped out his accordion and while we listened to his rendition of traditional music we were able to dance or admire the immensity of the surrounding mountainous backdrop of the Zillertal Alps. (thetravelmagazine.net)
La Villa, a member of Relais Chateaux and a beautiful Mediterranean Villa - style property of sun - blenched terracotta, sits on a hill with a stunning mountainous backdrop and is surrounded by typical gardens of dense, shrubby maquis vegetation. (luxurytravelmagazine.com)