I’ve seen ally and alley mixed up a number of times in recent months. I would hope that any of you reading this would know the differences, but just in case:
An ally is either “a person, group, or nation that is associated with another or others for some common cause or purpose” (noun) or “to unite formally, as by treaty, league, marriage, or the like” (verb).
An alley, however, is “a passage,such as through a continuous row of houses, permitting access from the street to backyards, garages, etc.”
Quoted text is from Dictionary.com.