How hould it change the game if ... ?

[NTJedi]

This change would never happen at this late stage. This change would be very unlikely to happen within a future version since castles might be damaged during a siege and capture, but rarely razed to nothing throughout history.

If Dominions_4 is ever developed you could request a damage/repair phase where the victor needs to repair some of the castle to achieve maximum defense and bonuses on the province.

A total razing of a castle where the walls turn to ashes would only be some high_end late game spell which is very rare and should probably have its own casting limitations such as only being casted during the summer at the cost of the casters life.