Preposition + hall

You could say either in this case. 'At' is probably more usual.

Hi again,

I would like to understand the use of 'at' in this case more deeply if there is a difference from 'in'. If the registration is held inside the hall, should it be 'okay' too to say "at the hall"? The reason i ask this because i was told it was in accurate to use 'at' instead of 'in'. In which situation where 'in' should be 'appropriately used'?

I hope i don't annoy you

If you think that you need to tell people that the activity is inside the building and not outside of the building (that is, if people might be confused about whether the activity is inside or outside-- and I doubt that most people would be unsure about this), then you should use 'in'. Otherwise, both 'in' and 'at' are common, and 'at' is more common. 'At' indicates a point