Study the information about the prepositions of place.
In/at/on + place
1. We say that someone is in bed/prison/hospital

We use at when we say that someone is at home/work/school/university/college

At sea is used in the meaning ‘on a voyage’. Compare ‘in the sea’ and ‘at sea’.

We were at sea for 20 days.
I love swimming in the sea.

2. We use at when we talk about an event.

At a party, at a conference
I saw Mark at a football match on Friday.
3. We use in and at when we say about buildings.

You can eat in a café or at a café.  (There is no difference.)

We say at when we talk where an event takes place.

We went to a concert at the Royal Festival Hall.
Remember: at the station/ at the airport

When we talk about somebody’s house/place:

I was at Sam’s house last week.

4. We use in and at when we talk about towns:

In with cities, towns, villages.
At or in when you say about the place as a point or station.

I would like to live in Moscow.
We stopped at (in) a small village on the way to Paris.