"Ultimately, it's an average-strength Castle, somewhere on par with Flax-Castle-Dungeon. After the Inferno patch was released, people started playing as Inferno much more often."
An average-strength Castle? No, Inferno is the strongest in WT!"
Demons are roughly like skeletons – quantity doesn't always translate into quality. People have killed 500 and even 800 demons. Because they are slow melee units, and therefore vulnerable to archers, fast or retaliatory melee units, magic, etc. Capturing and defending Castles can also be difficult.
Plus, as I said above, you still need to gather the demons, and it's not always possible; it requires skill and a certain amount of luck.
Added after 1 hour 53 minutes
Readme inferno - Wic tower, WT mm6, without trading: waiting for the Papacy =)
If someone claims that a certain Castle is the strongest, those who wish to refute this claim are given the right to choose a Castle of their own choosing.