AT     

1. We say that someone is at an event: 

2. We say at with buildings when we say where the event (film, concert ) takes place:

at a party - at a pop concert - at a conference - at a meeting 

Example: Where were you yesterday? - At the cinema.

                 Tom is at a party. The meeting took place at the headquarters.

3. We say at someone's house.

4. We say at for a place which is a part of our journey: 

We were at Bill's house last Thursday. We stopped at a very nice village. Does the train stop at Nashville?