Considering how difficult a direct assault was historically, I think the rate of fire is too low. Most cities if I remember correctly in the Punic wars for example were resolved by seige ie starving out the enemy, rebellion from within or by someone opening the gates. I know there are exceptions, but direct assault was not attempted very often precisely because it was so costly.
Apart from towers and later on boiling oil, there is very little in the way of wall defences other than troops. I appreciate that there may be coding issues that prevent this from being resolved, but personally, I'd like it to be tougher in the interest of "realism"