AK tuning and other modifications.

A moment from a real game: one mine was blocked, which controlled approximately 50% of the starting zone, but there was some questionable exit leading to another half of the starting zone... which, as it turned out, was blocked by guards. The solution to this situation was as follows: when playing as Necropolis, you need to build skeletons (as I understand it, in that half of the zone that is available) and break through the guards at the end of that passage (it doesn't matter that nothing is being built in the city), and that when playing as Necropolis, mines are not needed; just keep building skeletons and move forward.
Why isn't it stated in the rules that mines are not needed for Necropolis? Is building skeletons considered development?
There are many more such moments.