How difficult was "closing the gates" before a siege?

I'm reading this book, "The Origins of War" and it's talking about sieges going back to the Assyrians. One thing it mentions is how trickery was used to make a city think it was safe to then open their gates.

But, what I don't understand is what's so difficult about closing them? If there's a siege around your castle/city and you open the gates because you think they're gone, but they were just hiding and they start riding to your gate close the gate again.

I know I'm missing something obvious here, but I don't want to make too many assumptions about Assyrian or other pre-historic siege warfare.

My guess is that if a small group of soldiers from the siege army snuck in while the gate was open, they could kill the guards at the gate and keep it open long enough for the attacking army to begin pouring in.

http://www.amazon.com/The-Origins-Of-War-Alexander/dp/0813333024