Uh... Both?

Wait... Come to think of it, yeah Dumbledore might've already been dead but that long fall had to have cracked a skull...

Severus Snape, you're a monster...

How could he have survived the killing curse?

The Killing Curse obv. It kills instantly

The fall would have killed him, obviously, but he was dead as soon as he was hit by the killing curse.

The Astronomy Tower is stated to literally be the tallest tower in all of Hogwarts, if I remember correctly... so falling from it would certainly be fatal, but the same can be said of the Killing Curse- which does have the word 'killing' in it, and for good reason.

Didn't voldemort say in OOTP that ou have to mean it for ti to work. snape and dumbledore were friends so maybe he wouldnt have meant it only enough for him to fall and die

This wasn’t Snape’s first time using the curse. He knew what he was doing. He also had a lot of resentment towards Dumbledore built up, as we saw through their arguments around that time in the Prince’s Tale. Snape was probably able to channel that resentment so that he wanted to kill Dumbledore in that moment.

My one question is, how the hell did Dumbledore fall off the tower from all the way on top without breaking his own skull? Sure he was already dead, but I don't think that stops someone from bleeding out their head once they hit bottom. Unless someone somehow saw him falling and caught him, I don't see any other solution.

How do we know he didn’t break his skull?

It was never mentioned but I would think that he probably would've after that fall...