Tidewall Rampart
The Tidewall Rampart is the Tau's (So far) only fortification to exist (Drone turrets don't count). These prefabricated fortifications are delivered to a planet in compartments and assembled on the field. These things are a lot like a Weeaboo's lego set, as the Rampart comes in a few variants that can be mixed and matched, since these things are also semi-mobile due to the presence of anti-grav plates making them more akin to a hovering UGV (Unmanned Ground Vehicle) than an actual fortress. As such, the Tidewall Rampart is a whole load more versatile than their Imperial counterparts as, depending on the configuration, the Rampart can mimic any Imperial fortification like an Aegis Defence Line, Aquila Strongpoint or a Imperial Bastion on the move. Granted, this shit is orders of magnitude more expensive than their Imperial counterparts and it god damn explodes like a bomb if destroyed, so a loss of these Ramparts does deal a hefty economic and asset blow to the Tau.
Crunchwise, on the whole, these act more like T6 transports you can garrison your units in. Tidewalls do get <Sept> but can be embarked by any infantry, who can shoot from it. Firing Positions allows them to shoot even if the Tidewall is in melee "as if they had Big Guns Never Tire like a Vehicle". Unique to Tau, their forts can do an 8" Normal move (with FLY), as long as there's a unit embarked on it to do the Mobile Defenses action in the movement phase (even though embarked units normally can't do actions), meaning they're faster than walking and your models are protected by usually T6/Sv4+/W10 before they can be actually damaged, though at the cost of not being able to fire overwatch, advance, use abilities (or auras), and such.
All forts have a transport capacity of 10 infantry, models with W3 (only the Firesight Marksman) count as 2 transport spaces, and models with W4+ or more take up 3 transport spaces each. They cannot transport non-characters with W5+, meaning it's Fire Warriors, Stealth Suits, Crisis suits, Kroot Carnivores/Shapers, or Vespid. No more than 2 units can be embarked within them at the same time, but it's not like they'd fit together anyway.
Also, since they act more like a vehicle, one downside is that most destroyed forts can explode just like any other vehicle, but if it doesn't, then it remains on the field, wrecked, as difficult unstable exposed ground.
The Tidewall Rampart is currently made up of three parts, these are:
Gunrig
The Aquila Strongpoint of the configuration. The Tidewall Gunrig is the central and main part of a Tidewall Rampart, consisting of nothing more than a floating twin-linked Railgun mounted on a resilient Tidewall defence platform. The Gunrig constantly scans for enemies before unloading its projectile for the appropriate rail-fisting.
The platform shelters the operating Fire Warrior team while the turret rotates 360º for maximum efficiency and can engage both armoured targets and infantry using submunition rounds. The railguns themselves are operated by a Fire Warrior, such as a shas'ui from a Strike Team or Breacher Team via the control point set into the defence platform.
Crunchwise, it is the most powerful fort of the bunch, albeit with an increased price tag. W16 instead of the usual 10, and it comes armed with a Supremacy Railgun, which is 72" Heavy 2 S12 AP-4 D3+1d3 and the target suffers additional 1d3 MW on a successful wound, which it gets to shoot at BS5+ and with a closest enemy unit/vehicle unit (choose either) restriction unless it's got something <Sept> embarked on it. It may not be a hammerhead's railgun, but it's still a gun worthy of being called a railgun, and it's the best gun any fort is equipped with pound-for-pound. This will actually out-shoot a Devilfish (not that that's a high bar to pass), unlike a Droneport, if you're looking for a transport with excellent guns.
Droneport
As its name implies, the Droneport house several Tau Drones for both defence and recharging purposes. Droneports usually hover into position to cover the advance of the Tau army and serve as a mobile bunker for the warriors of the Fire Caste. It can collect and collate data to optimise the killing potential of the Gunrig, and the networked AI actively boosts the energy barrier produced by the Shieldline - enemy fire is simply turned back upon the foe.
While Fire Warriors man the defences Droneport provides, the Drones themselves are unleashed to strike the enemy from the air to provide covering fire; becoming the Droneport's greatest asset. These Drones are able to detach and provide suppresive fire, deploy their built-in shield generators to provide even more protection to the Tau or mark targets for powerful strikes. There are four Drones in total on each Droneport, which can be any of the following types: MV1 Gun Drones, MV4 Shield Drones or MV7 Marker Drones.
On the tabletop, you can take 4 Tactical Drones with it, and Docked rules mean the Droneport can fire their guns with its BS4+ and lack of targeting restrictions and ability to fire while engaged. It is equipped with Drone Control Systems, a drone controller by another name that only works while a unit is embarked inside, meaning it can use its docked gun drones and support another nearby drone unit.
Shieldline
The most vanilla of the Ramparts. These are your traditional fortress walls in which your little space weeaboos can hide behind its energy field cover and fire away. The field itself is considered by the Tau as a virtually 'impenetrable barrier' of glowing energy. The Tidewall Shieldline slowly moves across the battlefield on repulsor jets, virtually advancing forward and giving the enemy less chance and less room to hide with every inch of gained ground.
If the enemy tries to shoot the defenders behind the Shieldline they risk dying because their fire ricochets from the energy fields and sometimes even rebounding back into the foe's own lines. The Tidewall Shieldline also is the bulwark around which the fortification of the Tau networks are built – battlements protected by a Tidewall control platform and shimmering energy field.
Ruleswise, unlike the other forts in the family it (and its Defence Platform, if you take one) are the ONLY ones that can't fit models with more than W4, meaning they can't hold characters or battlesuits. The Shieldline has a 5++ and doesn't explode or turn into terrain when it dies.
Now a Shieldline can combine with one another to create a Tidewall Defence Platform. It can be useful if you want to get another fortification on the field without using another detachment slot. The smaller model may be easier to surround in combat. No shield, but does wreck (explode or terrain) on death, and otherwise is a Shieldline, making it absolutely beyond pointless - taking one of these instead of a Droneport has absolutely no purpose.