The essential prohibition against transportation in an alley is rabbinic: Eruvin 5b
Whether the prohibition against carrying within an alley [if the adjoining houses haven't merged with each other] refers only to introducing items from a house into the alley, or even to transportation which takes place entirely within the alley: Shabbat 130b-131a; Eruvin 23b, 35a, 74a, 89a, 91a, 91b