I've found that if you want to starve a city instead of assaulting it, it's still a good idea to build some siege towers. They can be useful in case the AI sallies out at the last turn: your units will have the towers deployed at the start of the battle and if you turn on Fire At Will, then the towers will start shooting the defenders that rush out through the city gates.



Another nice advantage is that the towers can be used as defensive structures. Infantry units placed between the towers will have their flanks protected, and if you play on the huge size setting then the units will also benefit from greater depth of ranks.