In my opinion, ghosts are one of the weakest units of level 3...
For clearing the surrounding area around the Castle, I started as early as week 2 (upgrading archers + halberdiers) and did it with minimal losses. For Necropolis, this process takes a little longer – the spellcasting skill is not yet well-developed, there is a lot of meat, but it is slow and lost in battles in the early stages more often than it is replenished...
Besides, there are often griffon lairs around the Castle, which is a plus for the humans, as it provides a few archons.