Flags with buildings

The Angkor Wat Temple is featured on the flag of Cambodia.

There are 5 National flags that feature a building on them.

Afghanistan Flag

Flag of Afghanistan

The building on the Afghanistan flag is featured on the emblem. It's a mosque (also called masjid) and it's a place of worship for Muslims.

Cambodia Flag

Flag of Cambodia

The Cambodian flag has featured a depiction of Angkor Wat (a city of temples built by the Khmer king Suryavarman II in the 12th century) in the center.

Portugal Flag

Flag of Portugal

The Portuguese flag contains 7 yellow castles with closed gates and 3 towers each placed on the coat of arms.

San Marino Flag

Flag of San Marino

There are 3 towers in the middle of the San Marino coat of arms that represent the citadels La Cesta, La Montale, and La Guaita.

Spain Flag

Flag of Spain

In the first quarter of the Spanish coat of arts (which is included in the national flag), there is a 3-tower castle with blue doors and windows representing the Kingdom of Castile.