The Castle Unit is different depending on which Empire you are using. Order has an Archidon, Chaos has a Dead, and the Elementals have an Air Elemental. The Castle Unit does not contribute to your population count.
The Castle Unit cannot be targeted or killed in any way. It may also have extra effects to its attack compared to the normal unit. The Castle Unit has a range that reaches just past the second Gold Row. It will target the first unit that comes into its range and will fire at that unit until it either dies or goes out of the Castle Unit range, at which point the Castle Unit will fire at the next unit that came into range and is still alive. This can easily be abused in the mid-game.
The Castle unit may also have a different appearance to its normal base unit. They usually have a certain Armoury option equipped. You cannot change what you Castle Unit wears, but you may have your normal units wear the same Armoury options as your Castle Unit.
The first Castle Unit costs 300 Gold. The second one costs 600 Gold, and the third one costs 1000 Gold (for all Empires).
Compared to a normal Archidon, the Castle Archidon has splash damage. When it fires at a unit, up to four near-by units will also take damage.
Castle Archers are fierce archers, and can target units the second the come into their range. Yet they will fire on the first enemy seen until they die or leaves their range.
You can tell which unit the archer is targeting, for their bow will follow that unit.
The Castle Unit for Chaos is a Dead. The Castle Dead wears a brown wrag that only reveals where the eyes would be.
Castle Deads don't do much damage upfront damage as a castle archer or elemental, yet the Poison they inflict can cause havoc- your men might not retreat alive. Castle Deads are a bit slow to respond when enemy units come into their range, and like castle archers, will target the first enemy seen, though that doesn't mean they can't poison multiple units. Dead projectiles are very hard to avoid.
The Castle Element does much more damage than a regular Air Elemental though it also has a slow attack speed, because of this a retreating unit can be hit quite far from the gold mines as the lightning attack is lock-on (so 'moves' with the unit) and locks-on while the unit is still in the 2nd gold mine range, but hits the unit later and further.
The Air Elemental can do splash damage like the castle archers and Deads. The Castle Elemental's attack used to not deal splash damage, but a patch altered it to splash burn. The castle Elemental's bolts can't be avoided, whatsoever and there is no way to 'tank' them with a second unit. However Cycloid protect can be used to 'block' the damage a unit may recieve from the bolts and Merics can try to heal units to keep them alive during the onslaught.
The Castle Units are usually used for base defense, turtling or preventing your economy from getting rushed in the early game.