From what I understand, Descartes wrote refutations to criticisms made of his Meditations. I'm just interested in what he would say to Wittgenstein, who I think has the most compelling critique of radical doubt.
Wittgenstein wrote in On Certainty, "If you tried to doubt everything, you wouldn't get as far as doubting anything. The game of doubting itself presupposes certainty."
According to Descartes, nothing is indubitable - we have to doubt anything and everything until we can clearly and distinctly perceive it.
But is everything really subject to doubt? Let's follow this to it's logical conclusion. Were we to doubt everything, we would have to doubt:
The meaning of our own words. Descartes would have to doubt the very meaning of each word in the sentence, "I think; therefore I am." He would have to doubt what the word doubt even means.
He would have to doubt the "laws of logic" so to speak. He would have to doubt every rule of inference that he uses to make his arguments. He would have to doubt the laws of thought themselves (excluded middle, law of identity, non-contradiction).
So to me, it would seem that subjecting everything to doubt is simply nonsensical. You would not get as far as doubting anything indeed.