how does Caesar continue to wield power over events even after his dead ? is he the true protagonist?

First, Brutus is the play's main protagonist. None-the-less, that's my opinion, and though it's a common opinion..... there are those who would agree with you.

Secondly, yes, Caesar does seem to wield a certain amount of power after his death.... but this power doesn't belong to Caesar the man, it belongs to Caesar the ideal. Caesar was well loved and respected, and that love and respect did not simply disappear for those who followed him. This is evident in the actions of Antony, who lies to save his own life, only to immediately put himself in danger. It also applies to Octavius, Caesar's heir, who refuses to be bullied or controlled, and acts in a way that would make his father (he was an adopted son) proud.