Wait, why can't I deploy?

One of my towns is under siege, but when the enemy attacks, it doesn't give me the option to deploy. This wouldn't be sp bad, if it weren't for the fact that my Musketeers and other troops are ON THE OTHER SIDE OF THE HUGE CITY. There's no way I'll be able to get them to the other side before the enemy does, and I can't hold them off with two units of Pikemen. I've had this once before, and I think it was because a spy was in my settlement, but this time it gives no message saying so.

Does anyone have any clue as to what's causing this?

Yep, you've an infestation of enemy spies holding the door open for their compatriots. Nasty, init?

Counter by leaving at one spy in your cities/castles purely for counter-espionage purposes. If you've got your guilds and generals' trait boosting your spies' skills, then you'll find each turn that you're capturing enemy infiltrators and that your intelligence forces are steadily acquiring retinues like beguiling bard, and exotic dancer.

Also, remember that the AI will find itself in your position when your spies do the same, and when you get the gates are open signal, rush your army in before they can effectively deploy.

What a game!

I guess I should have known. I took Florence as Spain, and the other Italian states didn't like that. First came massive assaults that I managed to beat off thanks to Merchant Cavalry destroying the cannons and Militia Spearmen holding the enemy long enough for the ballista towers to rip the enemy apart, and recently they've turned to what can best be termed a 'Spy-Spam'.

For the last 20 or so turns, it seems all the Italians (and France, for reasons I cannot fathom) have been throwing streams of spies at me. I put spies in all my Italian cities, and every turn they'd zap at least three of the buggers. It actually almost worked, as some of my cities got close to rioting due to the unrest.

I'm really impressed by the campaign AI so far. Using spies to cause unrest in cities, switching to using cannons when siege towers and rams aren't working and using the spies for sneak attacks. Never saw that in Rome.

Yep you got a spy that opened the gates.
Think it is suppose to simulate a sneak attack

I think it's already been mentioned somewhere else in this forum, and it certainly matches my experience, but the only guild the AI seems to pick up for its cities is the Thieves' one (which is invariably the first to be offered) - hence the onslaught of crafty skulk-in-the-nights that you're now facing.

I've noticed that when I get atacked I sometimes lack the time to prepare for battle. i think this is because the opposing army perhaps catches you off guard.

Possibly a bug with the AI/spy routine though. I've had my capitol (edge of my land so vulnerable) assaulted numerous times, ~1/2-3/4 of the time, the enemy had a spy, so no deploy.

However, everytime they assaulted as if there were no spy. I got the vocal message at battles start about some treacherous dog opening the gates, yet the enemy hits the gates with rams, uses ladders and towers. I almost wish I could point out to the prisoners that if they had only tried the handle, the front door was unlocked all along. :)