Inferno tactics.

1. Pitfiends can be deployed in the first week without much trouble, which I think is a plus. But the upgraded ability, as well as the upgraded Gogs, is questionable (I mean, why pay extra?).
This is a good, very strong unit, but their health is weak (although you wouldn't think so by looking at them - such a muscular, bloodied torso and a whip...).
2. If the map isn't completely depleted of resources, you can also deploy Imps in the first week. The upgraded ability is nothing special, although it can be useful in some cases.
3. Gogs are good ranged attackers, but I personally rarely deploy the upgraded ones - their attack on friendly units is annoying... Although +1 to attack is always helpful. If you're clearing out the surrounding areas with Gogs, Kal'th will help. I always take him at the beginning.
4. Devils have a very useful ability in many cases - retaliation. Overall, a decent level 7 unit. In any case, they inspire more confidence than the clawed bear, hydra, phoenix, and necromancer dragons.
5. Imps, I agree, are weak. Even large numbers don't help. Last time, I specifically tried to have my hero, a level 6 character, run around the outer areas, collecting these small creatures. I thought I could gather a legion of devils... In vain - they die with one or two hits.

Overall, the Castle is average. The upgraded abilities of the units are of questionable use, and I don't see any particularly "interesting" features in the heroes. The monsters are average, and they rely on numbers.