There are restrictions for this:
5.8. Restrictions on castles.
5.8.1. In the Conflux castle, it is forbidden to upgrade monsters of levels 1, 6, and 7.
When playing as Conflux, the starting hero is chosen randomly when the map is generated.
5.8.2. In the Necropolis castle, it is forbidden to build a necromancy enhancer.
When playing as Necropolis, it is forbidden to choose Galtran as the starting hero. If Galtran arrives as a non-starting hero, you CAN develop him, and for any castle.
Friend, the very fact of allowing necromancy is incomparable to any restrictions))) This is not like releasing skeletons, as in Fortress :D