By this > By this time
2 (Ambrosius and Uther) did ripe years attain,
Ambrosius > (Ambrosius Aurelius, fl. c. 440 AD, Roman emperor of Britain, Gaul and Spain under Honorius. He led the Britons against the Saxon invasion. Poisoned at Winchester by a Saxon)
3 And here arriving, strongly challenged
challenged > laid claim to
4 The crown, which Vortigern did long detain:
detain > hold
5 Who, flying from his guilt, by them was slain,
flying > fleeing
6 And Hengest eke soon brought to shameful death.