If it was marked wrong, it was probably because whoever added the exercise didn't anticipate (or doesn't like) the ellision of "around" to "round". I don't see any difference in habituality between "around" and "round", and "are you turning around?" isn't habitual at all.
Thanks for responding! I'm not sure I entirely understand, though. (Which may be a function of trying to learn a language from scratch). At least to me, it looks like the original sentence/translation requires the addition of "to do" as an auxiliary verb to form the question. Generally, when translating into english "to be" is also acceptable as an auxiliary (though it does require a gerund). I'm sure you're correct, I just want to figure out why. So:
Are you saying that because "An gcasann tú timpeall?" uses the verb "cas" (which I read as "to turn") that "to do" is the only acceptable auxiliary to add? And is that because "An bhfuil tú ag casadh timpeall" uses the verb "Bí" in the form "bhfuil" to create the equivalent of a gerund phrase? Is the practice to default to "to do" as the auxiliary of choice if "Bí" is not present?
Generally, when translating into english "to be" is also acceptable as an auxiliary (though it does require a gerund).
That might be true if you are translating from a language that doesn't differentiate between the present simple and the present continuous. It is not true when translating between languages that have both. An gcasann tú timpeall? does not mean "Are you turning around?", and An bhfuil tú ag casadh timpeall? doesn't mean "Do you turn around?". A fluent Irish speaker or English speaker would never confuse those two sentences. The difference isn't subtle, it's almost as significant as the difference between the past tense and the future tense.