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I've been playing as Inferno for a long time, and I can say that it's a Castle of average strength. It's not as strong as Castle or Rampart, but it can compete with the others.
What can I say about tactics?
My tactic is this:
I choose Nimue as my hero. I've concluded that Inferno's troops are more offensive than defensive, so Nimue is perfect, especially with advanced attack.
In the early game, I try to build Efreet. This is possible because they don't require many resources.
If Efreet are built in the first week, it's a big plus for Inferno.
Sometimes, when you need to buy two Efreet and go kill some monsters, you don't have enough money. There are many reasons for this. For example, there might be few gold mines on the map. When players see a treasure chest, they try to level up quickly, taking experience instead of money, which is a big disadvantage for Inferno. It's better to take gold from the chest in the first week, even though there's information that Inferno has the most expensive troops and needs a lot of gold. If a player builds a unit, they immediately buy it, which can also cause a shortage of gold. So, if you have some troops in the first week, plus two heroes from the tavern, you can defend yourself and get some gold and resources. A big disadvantage is buying the entire army in the first week, which can prevent Efreet from being built. And if you somehow manage to build them, you might not be able to buy them unless the market saves you. The market can save you, but you might as well write "gg" right away. I've concluded that you need to collect gold in the chest in the first week, and experience can wait.
With this approach, you won't only buy Efreet, but you'll also have some gold left over.
There's information that Inferno's troops are the fastest and only inferior to one other Castle, but I don't remember which one. I've concluded that if you get Air magic, it's better to choose it. If you use tactics, you'll be incredibly strong. I could write a lot more about tactics, but since there's a limit on this site...
I'll say this: if you think Inferno is a weak Castle, it means you always play as Castle, Rampart, or Necropolis. That's why it seems weak, and one mistake in development can ruin your entire game.
I've played against many people, and there have been wins and losses, but when a player sees me choose Inferno, they choose strong Castles themselves. This shows that the player is afraid of losing