Can you name for me a single example at any point in human history where a fortress was sieged without being surrounded or under fire? Because otherwise you aren't describing sieging, you are describing passing through the province.
The game abstracts things to the strategic level. You aren't positioning companies of soldiers on a battlefield, you are broadly deciding if armies are attacking or defending.
Yep, I am not describing siege, but I am not describing passing. I am describing situation when enemy occupies/controls/pillage province without bothering to actively capture key fortress, and I gave example of horde because it fits perfectly. If main city has fallen by surprise than fine, otherwise just burn/whatever towns without level_x forts, villages etc.
To be completely clear I am not thinking about wars in Low Countries with their dense habitation and networks of fortifications but about e.g. Eastern Europe and any other place where distances were vast
On the other hand even if we are talking about close blockade of fortress I disagree that attacker (sortie) should get defender (mountainous bonuses). At least it should be neutral "near fortress" terrain, but I think that attackers usually did not camp in range of defender guns (as many mentioned before) and set up rudimentary or better protection of their camp (ditch, whatever).
If you know historical example then enemy camped in range of defenders artillery I will be happy to change my opinion.
The point is that we are not talking about strategic situation - like e.g. when two armies collide (in such case IMO it would be better to keep random/depending on maneuver terrain anyway).
We are talking about very localized situation within few - maybe a dozen hundred meters at the start of game, and a few kilometers at the end.