I see lots of questions and claims about
first strikes in this thread, only half of which are true. Here's a rundown of how it
really works.
What is a first strike?
To understand that you must first understand what a
combat round is. When two units face off, they do so in a number of combat rounds until one of them has lost all its hit points. In each combat round, a random number determines which of the units hit the other for damage. If two units of exactly equal strengths are fighting, it's 50-50 who hits the other. If one has 3x the strength of the other, chances are 3:1 who hits the other, etc. The unit that loses the combat round takes damage, how much is also determined by a comparison of the unit strengths (see the article on combat for the full story). A
battle is a sequence of combat rounds ending when one unit dies.
With the knowledge of what a combat round is, we can now define what a
first strike is.
A first strike is a combat round in which only one of the units, the one with the first strike, can damage the other. Everything is calculated exactly the same as a normal combat round, except that if the random number indicates a win for the other unit, no one takes damage. So a first strike is a free chance to damage your opponent, before he gets the chance to hit you at all.
Note though that it's not free
damage to your opponent. You get a chance to hit him, and if you miss you don't do any damage. If your unit with strength 3 attacks his unit with strength 30, you're not likely to get anything out of your first strikes at all. Only difference from the normal combat that follows is that he can't strike back.
How many first strikes do you get in a battle?
This of course depends on your skills and promotions. An archer with the Drill I promotion has 1 first strike and 1 first strike chance. This means you will get 1-2 first strikes in each battle, where the exact number is determined by a flat distribution. A flat distribution means that each number is equally likely to happen. It's is
not a 50% chance for each first strike chance, which would result in a normal distribution. If you have 3 first strikes and 4 first strike chances, you are equally likely to get 3 as you are to get 7.
If both units have first strikes, both roll to see how many they would get, and the one who rolled the highest number gets to do the difference. E.g. one unit has 1 first strike and another has 2 first strikes, the second will get one first strike round in combat. If one unit has 1-4 first strikes and rolls 4 and another has 3-8 first strikes and rolls 3, the first one will get one first strike round in combat. Etc.
So what does this mean for my chances in combat?
Like Lexad says, first strikes usually help a stronger unit get even stronger. If your strength is higher than the opponent's strength, that's far more important than if you have a first strike or not. But if you are stronger already, your survival chances can increase dramatically, since the opponent can't damage you during those first rounds. Archers fortified in towns on hills with Drill IV will withstand a lot more attacks than those without. But likely City Defender is still better for that particular purpose, since it's specialized. I have no exact numbers to back this up though, it's simple gut feeling.