Milton is making a metaphor between the cannons to the birth canal of a female. Milton chooses to call the cannons "hollow engines". The human body is one of the most complicated machines. To be even more specific the female reproductive system is very detailed and unique.
Literally in the text Milton is saying the cannons will be tightly packed with gunpowder, but metaphorically the cannon is the vagina violently penetrated with Satan's future seed. The engines will be the vehicles for Satan's domination of Heaven and Earth.