From Halopedia, the Halo wiki
This munitions testing complex is a UNSC installation situated in the industrial wastelands on the outskirts of New Mombasa.
The complex was eventually decomissioned when safety concerns came to light. During the Battle of Mombasa, the city's skyline was clearly visible from the site.
The munitions complex is the in-universe locale providing the setting for the Halo 2 multiplayer level Tombstone, a remake of the Forerunner-themed Hang 'Em High in Halo: Combat Evolved.
Pillars seen in the testing complex. Note the word "Tombstone" along the base.