Which units would you like to replace/add?

Peasants pay for themselves within 3 weeks. So, you only gain profit by buying them. You can simply keep them in the castle and upgrade some of them to archers before the final battle, as it can't get any worse. And the hiring costs are negligible.

Added 3 minutes later

Does anyone want to introduce a poltergeist into the game? It also belongs to the spirits. And it's invisible too. Of course, not at level 7. Or some kind of ominous fog. I can hardly imagine what it would look like, but it sounds impressive... Ominous Fog... What do you think?

If you think about it, you can gather so many undead units that even 14 levels would be too few, let alone 7.