also, once again a wall does nothing against the mongols, what you think the mongols are gonna do if you don't let them in? They're gonna turn around and go home? lol, no you let them in because then certain death comes much faster, better to be sliced than to starve to death with your dead child's meat in your stomach know what I mean, only reason to have walls to so delay invasion so you can sit and contemplate surrender longer, that's what the chinese did accept they contemplated a little too long and starved on their thinking chairs, trust me they begrudged baghdad their swift deaths
Bro, don't even go there, I am a history buff, I have studied over 400 battles thouroughly and have spent 15 years on every major empire known to man, I can write a book about walled cities and have read battles over walled cities over and over millions of time. No one is good enough to face a wall cities without being at a disadvantage or take heavy loses in terms of ratios in casualties. Has never happened and will never happened. to seize a wall city is ALWAYS a disadvantage. There is only 2 reasons why people would take a wall city, 1 they have enormous numbers that they don't care about casualties or they know that the main Army is not present, any other reason is bad military skills period.
Do you have any idea what it cost to sustain an army camped out in front of a walled city while waiting for their water and food supply to be cut. huh, it could take up to ten years cause while you cut off the main supplies, they always have back up tunnels scattered that are hard to find and are not found for years sometimes. Once they truly cut off all routes they have to wait till their back up food and water to run out, and back up can last up to 7 years. All the while the camped up army is becoming malnourished and lazy.
Bro the way you are talking is delaying the invasion means days or weeks, ah no,no,no.. read your history brother, armies wait years and years, it ain't no short period. The Greeks waited ten years before conquering Troy, The Odyssey and the Iliads,... The Trojan Wars.