I think we're not talking about diplomacy here, but about which units to recruit into the army – unit A or unit B. For example, if you have two Dungeons and two Towers, the question is whether to recruit Scorpions or Nagas. This is where the pros and cons come into play.
The siege of a Castle is a separate matter. Castles are usually attacked when the army inside is much weaker. Or when the enemy has invested in Ballistics. I just don't think that with equal armies and without Ballistics, anyone would want to attack a Castle, or, having attacked a Castle, immediately send in Wyverns or Scorpions, which will be quickly killed.
I agree that Efreet are powerful; fire immunity gives them a significant advantage, and the fire shield is very effective! The fire shield makes them very unpleasant opponents; they are not easy to attack. I personally voted for them!
It depends on who you're playing against. If the opponent sends units from their Castle towards you, you can use Wyverns or Manticores to fly inside and block the archers. You need to anticipate how the opponent will act.
