Limitations of Catapults

"Lord Pasha, we should be able to break through before the next week!" 

Alexander was outside his tent observing the huge walls of Caira get pounded repeatedly by his catapults, when he received this 'cheerful' update from one of his chief artillerists, one who was responsible for coordinating fire in the right side of the wall.

The man was where for his daily update.

"Mmmmm! Good job. I look forward to it."

However, despite Alexander's words, the man sounded hardly that thrilled, giving a rather anemic hum. 

This was rather strange given that you would think Alexander would be a lot more excited hearing the close deadline.

The reason for this was revealed in Alexander's internal monologue, 

'One week is likely too optimistic. He was only saying this to please me. These walls may look like they are full of cracks and just about to shatter, but I suspect this is just on the surface. Stone is surprisingly hard to smash through.