most of the time the city just paid some money and was never sieged. citys normaly didnt have good walls or alot of people willing to defend them.This is true only depending on the army and time period in question. A general rule is that most medieval sieges (since the show most accurate reflects the high middle ages) ended with the city surrendering without a breach
they just didnt cared alot who would own the city because city rights werent different under different lords.
Castles on the other hand were alot better defended by people who cared.