If you haven't read my posts on Berseker Castle and Mage Castle, you may want to start there first.
They travel 2x faster than infantry! They have decent attack stats and provide a different type of attack than beserkers and mages. But they eat a lot so make sure you have enough food for them.Paladin City/Castle
Paladins are good because they are twice as fast with their horses. They are a great defensive unit type against magic attacks (mages.) They're great for raiding forest dungeons. A really awesome bonus is that you can recruit a baron in less than 10 minutes. I personally have at least 1 paladin castle on each continent for the purpose of recruiting barons.
I don't castle defensive cities unless the city is landlocked and on a secure continent. I.E. your alliance dominate that continent with little or no threat of attacks.Ranger City/Castle
In addition to being great defensive units, rangers are also great for raiding. I use rangers to raid mountain dungeon when they're not defending. Wood is used to recruit rangers so it lightens the demand on iron. Layout is very similar to Berserker Castle, I only change Foundry to Sawmill for wood storage instead of iron storage.Catapult/Ram
Good for castles on the same continent. Useless otherwise.Ballista City/Castle
Really awesome as a defensive unit. But can't raid so they are useless when not defending.
War Galleon Castle
If your alliance wants control of the water, you'll need WGs. They are awesome at destroying water castles. A full castle of WGs can destroy a 10k score castle in 4 hours. You can also raid sea dungeons/bosses with them. Their only downside is their food intake. So make sure you have a lot of food around.Sloop City/Castle
Like WGs, sloop can travel to other continents to help protect your alliance castles. If you have water castles, sloop cities/castles are a must!Mage/Frigate Castle
Mages are the best unit against ship. So Mage/Frigate makes a great offensive water castle.Knights/Frigate Castle
They provide a different type of attack. On ships, they loose their speed because they travel the same speed as frigate.Berserker/Frigate Castle
When not attacking, they are a great type of unit for raiding.Knights
Knights castles are ideally built on landlocked spots. But once you run out of spaces and your alliance control the navy, you can build other types of castle on water.Zerks
Very good at defending against magic (even better than paladins but not as good for raiding)