Unbelievably the fact that the Romans helped them build a city in 15 days might be the most historically accurate thing in this movie. Rome use to dominate in warfare back in the day. And the way they accomplished this was with heavy armor, lots of training and most importantly... engineering. The rules of ancient warfare meant that defending walls gave you a huge advantage. Like you need to have 3 to 5 times the number of troops to attack a properly built fort. And the Romans where the best in the business at building walls. Roman soldiers where said to be able to build a camp, while under attack, in as little as a couple hours. They built bridges, layed roads, made siege weapons on the go, nothing stood in their way. One time after the Germans told them that the Rhine river was the limit of roman power, Julius Cesar had his men build an overly elaborate stone bridge across 300+ feet of river in ten days, just to flex on the Germans. These guys where so serious about construction that each Roman legionary had as part of his equipment a shovel, alongside his sword and spear. And even weirder the way in which they fix the walls is somewhat accurate too. One thing the Romans really did right was concrete. They used to make everything with it, from the coliseum to the aqueducts, roman concrete was the shit. Much of it still going strong til this day! That's why roman buildings are so well preserved thousands of years later. So it is entirely possible for John Cussack's army to build reasonable walls in 15 days.
No, the real question here is why exactly did Jackie Chan have to build the city in 15 days? no one ever came to check that the walls where done. And even though they spent all that time building this great defense, all of the fighting takes place out in the open in-front of the walls. Building the city, like this whole movie, was a waste of time.