dragon > [Satan]
3 Bred in the loathly lakes of Tartarus)
Tartarus > (The underworld of Roman mythology; hell)
4 With murderous ravin, and devouring might,
ravin > rapine, pillage
5 Their kingdom spoiled, and country wasted quite: 6 Themselves, for fear into his jaws to fall,
for fear into his jaws to fall > [for fear of falling into his jaws]
7 He forced to castle strong to take their flight, 8 Where, fast embarred in mighty brazen wall,
fast > firmly
9 He has them now four years besieged to make them thrall.