"Castles are home to the Cleric and Knight hero classes. Castle armies are primarily composed of human men-at-arms, though these towns have also formed an alliance with the griffins and are under the protection of angels. With two types of ranged attack units and two flying unit types available, Castle-based armies are well equipped to quickly engage and defeat their enemies." RoE manual
Lighthouse - (Unique building. Gives +500 movement points for heroes travelling on water with a boat. The effect is cumulative. Doesn't boost enemies in Horn of the Abyss)
Shipyard - (Allows to build boats. Can only be built if town is adjacent to water)
Stables - (Unique building, gives visiting heroes +400 movement points per day for the rest of the week)
Colossus - (Grail building. Gives +2 morale to all player's heroes in addition to standard +5000 per day and +50% creature growth)
Some may find the information in this section subjective or irrelevant.
With two types of ranged attack units and two flying creature types available, Castle-based armies are well equipped to quickly engage and defeat their enemies.
Build and upgrade Portal of Glory by the cost of neglecting other dwellings and mage guild, or even recruiting other creatures.
Divide Archangels in several stacks (preferably in the multiples of three), so they can resurrect each other.
Brothehood of the Sword increases the morale bonus of defending creatures' by +2 in additon to Tavern's bonus. In other words, creatures defending a Castle towns with Brotherhood of the Sword have morale of +3. This becomes very useful if you want to mix troops from different town types together. However, because all Knights start with Leadership, the bonus is only valuable when using Cleric (or other hero classes) to defend the town.
Second most expensive creatures, Tower having the most. Hard to obtain the required 6 gems/week, without a pond or Tower town with a Resource Silo providing them.
The 6th level dwelling (Training Grounds) can be hard to build, as it requires 20 .
No units with immunity or resistance.
Most of non-upgraded units are significantly weaker than the their upgraded counterparts; upgrading them takes time and resources.