Why do building prices change in Rome 2 Total war

Only way to "split" an army seems to be having two generals. When you order a general to march to another general, you get a possibility to change troops from on army to another.

Compared to other TW games this seems like a huge drawback to be, because you dont seem to be able to recruit troops to undefended city if there is no general present nor can you leave troops to defend a city when your army marches on. City garrisons are usually not enough to defend a city and you need to carefully place your armies.

It does have a certain point, because roman armies were strictly hierarchical and there had to be a general leading it. It leads some inconsistencies tho, like in case your general is killed, you will be able to recruit a new one to that spot immediately.

http://forums.totalwar.com/showthread.php/84013-How-do-you-get-deploy-defensive-artillery/page3?s=4586ab6ec43eca9ce6bbe0860dbaa500

The last post in that page says that you only get the defensive artillery if the besieging army (AI) decides to build siege equipment, even though I'm all the way up to 25 BC as Rome and have yet to have a battle where they decide to use equipment, they use the 3-5 unit rush to the gate and use their torches.