‘At’ for place

At is used to talk about position at a point.

Example:

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.

Example:

The plane stops for an hour at Frankfurt. (Frankfurt=a point in a journey)

BUT

She lives in Frankfurt.

Let’s meet at the club. (Club=meeting point)

BUT

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.

Example:

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.

Example:
She works at SickKids. (SickKids=proper name for a hospital)

BUT

She works in a children’s hospital.

At is used to say where people study.

Example:

He is at the London School of Economics.

We use at with the name of the city to talk about a city’s university.

Compare:

He is a student at Oxford.

BUT

He lives in Cambridge.

At is used before the names of group activities.

Example:

at a party, at a meeting, at a concert, at a lecture