It's worth noting that if you retrain units in cities, they keep their experience. So if you've got two half strength units with, say, three Xp, instead of merging them into one unit with three xp, you could take them into a city, retrain them both and end up with two full units with three xp. Seems a bit buggy, but what the hell.

Oh, and another cheeky trick (cheesy exploit, but quite fun) - say you've got two units that are at 80 out of 160 strength in a city. Put them in the retraining queue, then merge them - and you'll end up with one unit with 320 men in it (the 160 men left, plus the two lots of 80 men that would have been added to the units when they were retrained.

It's nice to see a double strength phalanx stretching away into the distance...