Fortresses & the Wrong-Colored Bishop

A FORTRESS is a position the stronger side can NEVER break into — extra material just watches from outside the walls. The most famous fortress in chess:

ROOK PAWN + WRONG BISHOP: if your only pawn is a rook pawn (a- or h-file) and your bishop does NOT control the promotion corner, the defender simply parks the king in that corner — and it's a dead draw. Forever. Up a bishop and a pawn, and it does not matter.