Foul is she and ill-favoured, set askew:
Gaze not upon her till thou dream her fair,
Lest she should mesh thee in her wanton hair,
Adept in arts grown old yet ever new.
The opening line is dense and uncompromising. Rossetti uses  anastrophe  to start, that is, she inverts the usual word order to place the key adjective first. ‘Foul’ is hard-hitting, with a  fricative  ‘f’. The next adjective, ‘ill-favoured’, is an archaic form of ‘ugly’. That she is ‘set askew’ could refer to the sinfulness that distorts her beimg. She lacks the symmetry and balance of something good and pure. 
 The line itself is rhythmically choppy and unsettling. 
 It is an ominous start; a remarkable example of Rossetti’s skilled craftsmanship.
Babylon the Great