No idea on triggering events, but for the most useful Skills out there:
Direct control allows your army to be more flexible with placements and you no longer have to micromanage 3-4 units and instead dump all your AP on a single most effective unit. Don't over-do it, but an army with 2 Warlord will wreck most enemies with ease.
It's always annoying to have to fight past 50,000 troops, break down a gate, or climb over two walls just to take over the city. Wouldn't it be faster to just lure the leader out, surround him, and capture him to end the fight? This is only applicable vs Low INT officers btw, which is why I always spare the stupid ones (INT<60) and execute the smarter ones if they refuse to work for me (INT>60). I took out all of Yuan Shao's sons with this strat. They never survived the ambushes. And of course, I let them go upon capture.
Sometimes you want the info to be exposed for a long term, and this helps you just that. It's not that crucial, but helpful to keep tabs on your enemies. Spy lets you plant spies to keep the information displayed for a longer time than usual.
Another great skill but require the strategist to be close enough in harms way unlike Entice.
A chance to ignite a flame on hit from range on top of the ranged damage? I definitely like this when the wind conditions favor it. A fire is like another unit on your side, blocking movements and dealing damage. Take note that even Horsebow Units can launch Missiles.
Very useful to maintain high experience on veteran units, hastening their upgrade.
Rallying seems wasteful when used on just one unit, but when set up right, rallying five units at once can be immensely rewarding. You have thirty days to finish the fight. Why not spend the first few days rallying 5 units while the rest of the veterans get into position? Also, it is a very good way to increase strategic experience to unlock the immensely useful Warlord skill.
Another powerful item effect to note is 'Red Hare'
The 'Retreat Certain' effect made sure any escape is guaranteed in battles, this is extremely powerful for harassment raids. You need to remember that any development order requires a month to complete, if an attack occurs, it will cancel all orders for that city. So what do I do? I bring in Lu Bu with just 10,000 Horsebows, attack and employ ambush tactic to remain out of sight. If I see targets of opportunity, I walk out and destroy, and reserve enough AP to return to Ambush position.
Do this for 30 days and the defenders would've claimed a very hollow victory. Your budget is 20,000 food and 10,000 men.
They lost a month worth of supplies consumed by their entire garrison, and their entire work for the month is canceled. I did this 5 times in a row, and eventually the defense just crumbled with the City Order at 33 due to the constant battles.