At and In:

"It" is used for smaller localities and for more extensive localities. "At" is used for a point of time, for a period of time

Examples:

He lives at Clifton in Karachi

They lived in Gujranwala but I preferred to live in Lahore

He will come in four days

The train arrived at the station at 9 a.m.

The main train is due at 7 p.m.

Many a battle was fought at Taxila in India

We started at ten in the morning