The Chimeran gun tower is a form of Chimeran technology that is used to protect the European coastline from invaders. Over the course of the two years that the Chimera had occupied Europe, they built several of these gun towers all over the northern coast of the continent, making any kind of aerial invasion impossible. They only appear in Resistance: Retribution in the first mission in Rotterdam.
Maquis Intel stated that Rotterdam was clear of any defenses, and so for a brief time there would be an air corridor for the British to send their troops over to Europe. But two weeks later when Operation Overstrike began, the coastline of Rotterdam had already been fortified by the Chimera, indicating just how fast they can build. When the first squadron of British troops attempt to fly over Rotterdam, many of their VTOL aircraft are shot down and the survivors are scattered all across Rotterdam. Steven Cartwright, first squadron's commanding officer, wishes to destroy these gun towers to ensure that Rachel Parker's second squadron does not meet the same "welcome" the first squadron received.
Colonel Roland Mallery and Cartwright managed to destroy two of the gun towers while Lieutenant Raine Bouchard destroys a third. Mallery and James Grayson together infiltrate and destroy the fourth and final tower, clearing an air corridor for the second squadron and so Operation Overstrike can begin.
The other gun towers are likely destroyed over the the next two years when the British and Maquis are retaking Europe. It is unknown if there are any other towers anywhere else in the world, but nothing else is mentioned about them after Operation Overstrike and so it is likely that none have been constructed.
In the Intel documents that are scattered across the level, Grayson mentions how these gun towers are different from any Chimeran technology he has ever seen, and believes that the Chimera are capable of so much more than they have shown. Another possibility is that these gun towers aren't even Chimeran, and that another alien race developed the technology that was used for building the gun towers.