Answer:
But when he once attains the upmost round,/He then unto the ladder turns his back"
Step-by-step explanation:
Brutus thinks Ceasar will ruin Rome if he becomes king. Brutus refers in these lines to ambition and power. He means that when Ceasar reaches power, he will forget about all the people, being Brutus himself one of those, who have helped him reach that the crown. Therefore, Brutus fears Ceasar will not be the same; he will become disloyal to his followers.