Definition of "perpendicularity"

In their spare moments the animals would walk round and round the half-finished mill, admiring the strength and perpendicularity of its walls and marvelling that they should ever have been able to build anything so imposing.

1943 November – 1944 February (date written; published 1945 August 17), George Orwell [pseudonym; Eric Arthur Blair], Animal Farm […], London: Secker & Warburg, published May 1962