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Well, if we're talking about levels, I'll pass – there's too much choice of creatures I'd like to see in my army... But as a rule, I combine units from two Castles (it's more effective, and morale is more or less okay, especially since I try to take Leadership), and since I mainly play as Bastion, I can write down my favorite combinations in each case:
Bastion+Castle
- Elf
- Unicorn
- Dragon
- Archer
- Angel
- Champion
- Pegasus/Griffin
Bastion+Tower
- Elf
- Unicorn
- Dragon
- Titan
- Naga
- Djinn
- Mage/Pegasus
Bastion+Inferno
- Elf
- Unicorn
- Dragon
- Devil
- Ifrit
- Gog
- Pegasus/Lord/Cerberus
Bastion+Necropolis
- Elf
- Unicorn
- Dragon
- Death Knight
- Lich
- Vampire
- Bone Dragon/Pegasus
Bastion+Dungeon
- Elf
- Unicorn
- Dragon
- Dragon
- Manticore
- Medusa
- Pegasus/Beholder/Minotaur
Bastion+Stronghold
- Elf
- Unicorn
- Dragon
- Cyclops
- Rukh
- Monster
- Pegasus/Orc
Bastion+Fortress
- Elf
- Unicorn
- Dragon
- Lizardman
- Wyvern
- Gorgon
- Pegasus/Snake/Hydra
Bastion+Conflux
- Elf
- Unicorn
- Dragon
- Water Elemental
- Phoenix
- Mind Elemental
- Pegasus/Storm Elemental
Whenever possible, I try not to combine Elves with Inferno and Necropolis – no offense, they're good Castles, I just don't think they look very good together, IMHO. As you can see, I try to take at least 2-3 archers and more flying units, but I always have a strong infantry reserve – younglings :)
Of course, it's all IMHO ;)