Cairo is a location in Serious Sam 3: BFE.
In Serious Sam 3: BFE, Cairo is in ruins and there are no traces of human life left in the city. This suggests that the city has been destroyed by Mental, giving a grim outlook for the rest of Earth's cities. Ammo and a few destroyed M1A1 Abrams tanks suggest that the city was recently under a siege by Mental's forces, which it seems the humans lost. The siege might've been unexpected, as abandoned cars can be seen scattered throughout the city. Minigun turrets, originally meant to fend off hordes of Mental's forces, can be found throughout the city. They are still active, making some locations in the city quite dangerous for anyone foolish enough to still be in it.
Mental's forces have transformed parts of the city for their own needs. Antaresian Spiders have turned some parts of the city into nests for them to lay eggs, both indoors and outdoors. Major Biomechanoids are emplaced in certain spots in the city in order to shoot down any air support that flies into it. Scythian Witch-Harpies exist in the suburbs, waiting on the top of buildings for prey.
Cairo plays an important part early in the game. Sam and the rest of Alpha Squad are sent into Cairo to rescue Professor Stein in a museum who have the intel about powering the Time-Lock, but are shot down soon after they enter the city. Sam manages to enter the museum, which has been transformed into a nest for Antaresian Spiders. Sam is too late to save Stein, but he does save the research that he found. After uploading the data, Sam escapes the museum.
When he gets out of the Museum, Sam is contacted by Garret, who will extract him from Cairo with his AH-64 Apache. Sam gets to the extraction point, a large mosque, but before Garret can land, the helicopter is shot down. After defeating a Major Biomechanoid, Sam hurries to the helicopter's crash site, located deeper in the city. As soon as he approaches it, a strange monster takes control of the helicopter, transforming it into the Technopolip. Sam runs from the Technopolip, shutting down Minigun Turrets while killing opposition. He finds an XPML21 Rocket Launcher near a gate, and uses it to destroy the Technopolip. Instead of calling in for another helicopter, Sam decides to walk out of the city.
After destroying the Technopolip, Sam goes through the gate, where he is attacked by Scythian Witch-Harpies. Sam kills them, then continues his fight through the city. Eventually, he finds himself in a large field with ruined buildings everywhere and a huge flock of Harpies that have been teleported in. Right after they're killed, an Alcor Class Warship appears and attempts to kill Sam. Thanks to some quick thinking, Sam manages to do enough damage to the Warship to make it retreat. With the area clear, Sam finds a car, and uses it to drive out of the city.
Based on the game's ending, it can be assumed that Cairo was completely destroyed when Earth's moon landed on it.