Yeah, we had a good discussion about this earlier. There are a number of approaches that make sense: no working oil at an open gate or at least a delay before the oil starts to work (since it is a surprise attack and they would have to stoke a fire rapidly to heat the oil.) Another suggestion was that the defenders should start out randomly scattered about the walls or within buildings and not be positioned properly at the start. Afterall, when the AI sallies the player is stuck in a position of the computer's choosing.

The usefulness of a spy to open gates is certainly terribly diminished by the oil issue. Most defended walled cities that actually succumbed to siege actually fell to treachery from what I've read. While bribing a garrison is a fair representation, a spy opening the gates could be made a bit more "real."
Good points. Wonder if they're going to fix this in the next patch?

But really, that would make much more sense, and maybe have the attack during nighttime, haven't seen any of those battles in SP. It is, after all, a surprise attack.

And yes, it is suicide to attack the gates, more so if they have a phalanx blocking the gateway. If they don't, well, you CAN storm through the oil but it will cause casualties.