Siddhant told me that he lives  on  the fourth floor of this building.

Can't we use 'at' on place of 'on' in this sentence?

Siddhant told me that he lives on the fourth floor.

Here, 'on' is used and 'at' cannot be used. Explanation for the same is;

'At' is used when we describe an area within a larger area. It indicates a point or place occupied in space. For instance,

• She is at Gurgaon in Haryana.

• They were at the playground sharp at 6am.

'On' as a preposition talks about a space upon something. It means to remain supported or suspended. For instance,

• He was on the dance floor for more than an hour;

• They kept the apartment that was on the seventh floor.