At is used to talk about position at a point.
It is very hot at the center of the earth. (Center of the earth=position at a point)
Sometimes we use at with a larger place, if we just think of it as a point: a stage on a journey or a meeting place, for example.
The plane stops for an hour at Frankfurt. (Frankfurt=a point in a journey)
She lives in Frankfurt.
Let’s meet at the club. (Club=meeting point)
It was warm and comfortable inside the club.
We use at before the name of a building, when we are thinking not of the building itself but of the activity that happens there.
Eat at the Steak House – best food in town.
Sorry I could not take your call; I was at the theatre.
We use at with proper names used for buildings or organisations.
She works at SickKids. (SickKids=proper name for a hospital)
She works in a children’s hospital.
At is used to say where people study.
He is at the London School of Economics.
We use at with the name of the city to talk about a city’s university.
He is a student at Oxford.
He lives in Cambridge.
At is used before the names of group activities.
at a party, at a meeting, at a concert, at a lecture