Question

That lowliness is young ambition's ladder,
Whereto the climber upward turns his face.
But when he once attains the upmost round,
He then unto the ladder turns his back,

Answer 1

The ladder Brutus mentions in this excerpt is a metaphor for

Caesar's climb in status and power
Caesar's desire to remove all other leaders
Caesar's possession of little ambition
Caesar's weakness for leading Rome