This is a folly. That is to say it’s “a whimsical or extravagant structure built to serve as a conversation piece, lend interest to a view, commemorate a person or event, etc.; found especially in England in the 18th century.”*

Or, in this case, found in New Orleans in the 21st century.

Metairie Cemetery
New Orleans, Louisiana
photographed 4.24.2018

* as per dictionary.com.