This symbol places the noun archway in an enclosure. Hence, it is an ideograph for the noun city gate.
Illustrative Text Reference:
Harappa: Tablet in bas-relief: H-1840 B: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 258: New material, untraced objects,
and collections outside India and Pakistan: Suomalainen Tiedeakatemia.