Back of Block-15 from top of Block-3 stairs

I am on the landing at the top of Block-3 stairs.  The girls' toilet block is just visible on the left.

My 1CG form room was the ground floor classroom at bottom left of Block-15 (with the bourgainvillia).

I wonder why the ground floor windows are so different?  The third window from the left (with the keystone) looks like a chapel window from inside.

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