It depends on how we're defining 'effective'. As EnclavedMicrostate says in their compilation post, "Well, yes, but actually no, but also yes?" More can always be said on the matter, of course, so further posts will be welcome; however, as "was the Great Wall effective" is one of the more common questions about it, we do have several previous posts on the matter:

• u/cthulhushrugged has a recent answer focusing on the Mongols (and by 'recent', we mean 'written this year');
• while u/EnclavedMicrostate has an even more recent post about the Wall. Also in the same thread, I have a link to one of their previous compilations about the same question.