'At' is a point position-- it has no dimensions, so to speak.
'In' is a location within, enclosed by, inside something else.
I bought some towels at the department store (it is far away, and I am just speaking of its point location in space; I am not thinking about its interior, as of course I did not buy the towels outside the store) and I put them in (or into) the bathroom (that is, within that room rather than leaving them on the kitchen table or outside the bathroom door).
When do I need to use 'at'?
Why musn't i use 'at' in this sentence?
I put some towels at the bathroom.
Why is this sentence correct?
I put some towels in the bathroom.
'in' is short for inside, so when I say, "I'm in the store", I am expressing that I am inside the store.
'at' refers to a general location, so when I say, "I'm at the store", I am expressing that I could be in the store or outside the store. 'at' is a great preposition! Unlike 'in', which tells people exactly where you or something is located, 'at' is vague. It refers to a general location.
Use 'at' to talk about a general location, and use 'in' to talk about inside the location.
I put the towels at the bathroom. (Not OK)
I'm standing at the bathroom door. (OK)
I could be inside the bathroom standing in front of the door or I could be outside the bathroom standing in front of the door. Both meanings are possible with 'at'. It's a great preposition.