A lioness hath whelped in the streets,
And graves have yawned and yielded up their dead;
Fierce fiery warriors fight upon the clouds
In ranks and squadrons and right form of war,
Which drizzled blood upon the Capitol;
The noise of battle hurtled in the air,
Horses did neigh, and dying men did groan,
And ghosts did shriek and squeal about the streets.
In 1601 (or thereabouts) Shakespeare wrote Hamlet, and in the very first scene Horatio describes exactly the same night:
In the most high and palmy state of Rome,
A little ere the mightiest Julius fell,
The graves stood tenantless and the sheeted dead
Did squeak and gibber in the Roman streets.
That Hamlet's version is shorter is of no great consequence. Of course, Julius Caesar is going to spend more time on Julius Caesar. What's interesting is that Shakespeare is clearly working over his own original, finding the flaws and improving. You can almost hear him muttering "Did shriek and squeal? Did shriek and sqea... Did squeak! Why didn't I think of that in the first place? Squeak and um... gibber."
The grave as mouth was a favourite device of Shakespeare, for example "the grave doth gape/For thee thrice wider than for other men". But here he has consciously cast that aside and turned the tomb into a bijou rental property. The graves stand tenantless, making it a touch more Christian.
You can see Shakespeare fiddling with his words of 1599, dwelling on them, improving them, working them over. And you can see how he worked with the sounds of squeal and shriek. And you can see how he considered and changed his metaphor from grave as mouth to grave as boarding house.
It all relates back to Shakespeare as a worker rather than a genius, which I have written about before here and here.
For more on Shakespeare's attitude to death, see this old post. For the moment, I shall simply add that Calpurnia's "I never stood on ceremony" was the first time that anybody had ever stood on ceremony. Stand on already meant pay attention to (as in "Stand not upon the order of your going" which means "don't worry about who leaves the room first"), but standing on ceremony, that was Shakespeare.
The Inky Fool asking for proof-reading help