[Renojustin]

Sieges can last more than a year. At that point, defenders of the fort are usually down to eating bugs, shoe leather, and each other.

All Saruman had to do was lock them up in there for a few weeks and Rohan would have died. Then wait for Gandalf on the fifth day, and kill his ***.

A besieged city would be much, much worse. Riots, murder, disease, and cannibalism would run rampant... and then the really bad stuff would start.

So you go right ahead and hole up at Helm's Deep, buddy. I've got dibs on your leg!