We're familiar with the metaphor of the ladder of success. Caesar, like many politically ambitious youths, climbed the ladder of success by cultivating his popularity among the “lowliness,” the lower classes. Brutus thinks that Caesar, like the others, will turn his back on the plebeians once he’s achieved the heights of power.
But there’s a cleverness in Brutus’ choice of words. “Lowliness” can also mean humility, and it’s through his display of humility that Caesar developed his popularity among the plebeians. Finally, “base degrees” can also mean lower steps—the bottom rungs of the ladder in the metaphor.