From Lexicanum

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Stronghold Class Commerce Vessels belong to the Demiurg and are extremely well powered, large, and technologically advanced. They are factories or ore-processing vessels and are often bases which are moved between systems and asteroid belts. They are also bases for a fleet of inter system asteroid mining pods, haulage flyers and prospector probes. Strongholds are often seen in uninhabited systems with fleets of small ships moving resources to and from the vessel and a planet or asteroid, with the prow facing the star. Most of the time the vessel will disengage if challenged but some will attack with a dangerous ferocity.

They are slow but well defended, especially at close quarters, and have a good capacity for launching retrofitted mining vessels as attack craft. The ship also uses an array of complex electro-magnetic fields around which are accelerated towards the rear of the hull in order to give some form of fast increase in speed. This process is not understood by the Adeptus Mechanicus and they would very much to get their hands on an intact vessel to analyze it.

Sources

• ¹:Online Tau Fleet List

Kor'vattra - Tau Navy
Type	Design	Class
Capital Ships	Tau Battleships	Gal'leath - Explorer • Or'es El'leath - Custodian
	Cruisers	Il'fannor - Merchant • Lar'shi - Hero • Il'Pourri - Emissary • Lar'shi'vre - Protector
Tau Escorts	several classes	Kir'qath - Defender • Skether'qan - Messenger • Kass'l - Orca Gunship • Kir'la - Warden Carrier Escort • Kir'shasvre - Castellan Heavy Escort
small craft	air/space bombers and fighters	Manta Class Bomber • Barracuda Class Fighter • Remora Drone Stealth Fighter‎
mercenaries and allies	Kroot vessels	Warsphere
	Nicassar vessels	Dhow
	Demiurg vessels	Stronghold Class Commerce Vessel • Bastion Class Commerce Vessel