Necropolis or Castle (Which one is stronger?)

The Devil is a life-saver.
The Harpy is the most powerful ranged unit, not counting the Titan (and those who attack in a group).
The Cerberus has noticeably more health, which means good protection against magic.

I agree about the Devils... plus the Ice Golems... The Green Slimes are one of the slowest ranged units (only Ballistas and Wasps are slower), going anywhere with them is a torture... and Cerberuses - people won't attack you with magic, but with their hands and feet... so, Ghosts are more useful.