In the updated diagram, the corridor is 15 ft. wide, because 15 ft. is the diagonal distance across two squares: 5 ft + 10 ft.Geron Raveneye said:How do you figure that? The corridor remains at 15' width all the way through (general inaccuracies are due to my drawing skills rather than geometric differences), and at least in the DMG, a Huge creature uses a space of 15'x15'. So why should it have to squeeze through the corridor, which is at the same width diagonally as it os orthogonally to the grid? I don't follow.

A Huge creature occupies a 15 ft. by 15 ft. square. This is 3 squares across the orthogonals.

*However*, it's also 3 squares across the diagonal. By any 3.5 metric, the diagonal space of a Huge creature is not 15 ft. but (at least) 20 ft..

Since our example corridor is only 2 squares (15 ft.) wide diagonally, it is too narrow to contain the diagonal size of Huge creature.

(And before anyone asks, yes, you can trivially achieve the same effect with a hex grid as well)