From the info that is there, it seems that a good way to increase the possibility that defend triggers is to give it to units that have:
-severely less starting HP (so that they are less likely to ever be faced with an attacker that has less HP than them)
-have stronger stats (So that they can survive despite the large negative HP bonus; only in the case that the actual stats do not factor into the overall probability that defend is triggered, which is something that has to be tested)
-tend to fortify a lot (not sure how this can be assured; perhaps an immobile unit, or units that are preplaced deep inside enemy territory)
Although its up to unit makers to make a defend animation or not, it should be an item in the tutorials forum, or a sticky in the unit creation forum, that explains how you can ensure that the probability it is triggered will become greater (so that people can adjust the stats for the units they want to use it ingame)
Re the actual odds:
when there is more info i will try to actually calculate to a degree what they are. But keep in mind that probabilities are not something which you can see instantly materialise in reality. The example with the coin is one thing, but in abstact environments (eg the game) this means that even if the equation was such that the probability ALWAYS was 1 in x, this would not mean that if you run a test an x amount of times you would definately see it happening once. You could run the test 10x times and do not see it happening at all, or run the test 2 times and see it happening twice. A probability of 1 in x means that if you had run the test for a number of times that nears an infinite number of times, the probability would tend to be 1 in x.
But computer games do not seem to use pure probabilities, due to this problem. Instead they use probability variators, so that they can ensure that 1 in x happens ussually almost once in an x number of attempts. This is why you see the spearman defeating the tank; it is a balance program for the probability (at least i think so). So the actual equation that computes the probability most probably has such a variator, that would probably factor some elements of the rest of the equation so as to create a balancing effect.
Im not sure how trainer programs (that are meant to calculate the probabvle outcome of a battle) are built. PErhaps someone who has built one for civ3 can share some info?
