Been re-reading Battle of Helreach,
and while its an awesome book, I still cannot get my head around the idea of the feasibility of assaulting something as large and complex as a Hive city.
Supposedly the attacking force was in the 'millions', but by contrast a Hive supposedly has populations in the multiple billions.
True not everyone is a soldier, but the harsh realities of daily life in a hive complete with gang-wars and constant warfare and such would mean the at by in large most citizens would be armed, and tougher than your average joe.
(they live on a planet whose sole purpose is to make weapons....Im sure arming the populace isn't too much of a problem.)
Secondly....complexity. Take the worst instances of brutal house to house fighting and multiply it by a zillion.....consider the fact that sniper holes are not only in the buildings around you, but (due the the multilayer nature of a hive)
....the buildings hanging from the ceiling above you and from below as well.
The orks were outnumbered...like 10,000 to 1...and attempting an offensive battle into the best defensive terrain imaginable.
Thirdly....looking at the map included with the story:
We can see that indeed the entire battle takes place on only one level of a hive tower that stretches to the clouds. Say they actually captured that level....What next? One down....a thousand to go?
In the end I will admit that the actual story....and most stories involving Hive cities does not match up with the map provided. The author portrays Helsreach as a single level, open-air city with a wall around it as opposed to the enclosed towers in 40k artwork.
In fact I have never seen any author attempt to write about a Hive city battle with the traditional enclosed spire design.