BC is an exterior side.
zz is an interior side.
abcdefghijk is the trace of the enceinte.
bcdef is a bastion.
zdef is a demi-bastion.
de is a face of the bastion.
ef is a flank of the bastion.
fg is the curtain.
bf is the gorge.
(Two demi-bastions with the connecting curtain make the bastioned front, defghi.)