The idea was old, dating back to metal scaffolding towers sunk into the mouth of the Thames River during the Second World War, in the 20th century. The modern version of the Mauncell Towers are substantially more sophisticated, but largely rely on the same basic premise.
The Estuary Tower is built on a barge like platform which allows it to be semi mobile. Once a tower reaches it's destination, the barge is flooded, and settles to the bottom. The most common tower format has a single barrel like cylinder holding up its superstructure. Other variations include a lattice shaped cage, three exposed beam legs, and multiple barrel columns.
1. The Heli-Deck
The top of a tower is a wide open area designed for VTOL and helocraft to land and take off from. The outer perimeter of the top deck is studded with sensors and weapon systems. Given the confines of space and logistics, these are typically energy based lasers and masers, and are for anti-aircraft and anti-personnel operations.
2. The Command Deck
The command deck has the command information center, communications suite, and is the nerve center of the tower. Estuary towers are typically part of a network, and will work with each other as well as with surface ships, flying ships, and underwater assets. It isn't uncommon for raft cities, seacologies, or demi-ark sized vessels to keep a number of towers in their flotilla for protection purposes. There are still many places in the world where there is still piracy.
3. Action Deck
The Action Deck houses the weapons and defensive systems of the tower, and this varies from tower to tower, depending on what it's mission profile is. There are anti-ship and anti-mecha towers that are armed with guided missile launchers, rail guns, and high energy beam weapons. Anti-submarine warfare towers have an additional sensor suite, attached to underwater acoustic drones, mini-hunter killer drone submarines, and computer guided depth charge launchers and other deterrents. There are rumors of 'ghost towers' that utilize active camouflage systems, stealth technology and shrouding systems to hide in plain sight while acting as surveillance and recon centers.
The residential deck houses the crew quarters, mess, and common areas. The towers typically have small crews, and are typically spartan in accommodations.
The engineering deck houses the water recycling system, food storage, and power generation systems.
6. Vertical Pier
The bottom level of a tower is the 'vertical pier' and is used for dropping https://strolen.com/viewing/7696hardsuits into the water, and offering service and boarding to surface craft. Towers with a single column base don't have a vertical pier section, instead have a hollow core that extends down to the barge underwater. Most of this column would be flooded for weight and stability, but the water can be pumped out for service and access purposes.
Usage: The estuary tower is a manned, somewhat less portable version of the perimeter pod, allowing groups to create flexible defensive lines across watery terrain without having to field surface ships or flying ships, both of which are expensive, and require larger crews. Most of the functions on a tower can be automated, and they can be manned by autons and other robotic crew.
Additional Ideas (1)
The Rocket Tower is an extra-terrestrial version of the Estuary Tower, and is designed for use in space, or on planets other than Earth. The main body is pressure sealed, and has facilities for docking with spaceships, and replaces the barge with A-pods and engines, allowing the fortress to move at a slow speed. These are not found on Earth due to the large size of the engines required to move in one g gravity. They are more common on Mars, the Moon, and on various moons through the solar system.
These towers aren't all designed for military operations, many are used for scientific and exploration purposes, as mobile bases, and can carry useful equipment such as industrial and excavation mecha, cranes, drone control centers, and crew quarters.