  An automorphism of a building is called uniclass if the Weyl distance between
any chamber and its image lies in a single (twisted) conjugacy class of the
Coxeter group. In this paper we characterise uniclass automorphisms of
spherical buildings in terms of their fixed structure. For this purpose we
introduce the notion of a Weyl substructure in a spherical building. We also
link uniclass automorphisms to the Freudenthal--Tits magic square.
