The problem with this is that everyone would just pick drill every time. bonuses while defending are naturally going to be more useful on units that are designed to defend. This is the main reason I didn't choose to add a defensive choice and instead split it on city-attack and unit-attack. Well, the thing is if drill is remade as I suggested, you're not actually going to give up much by picking it over shock, and that extra 60% bonus versus cities might actually help you if you decide to land a few punches on the city. That being said I consider both 20% bonus vs cities and +25% flanking-bonus to be minor effects, more there for flavor than anything else. You get +10% CS from the promotion, plus your choice of a minor flavor effect. About the issue with fortified units that you keep bringing up, fortify isn't actually very useful, mostly because you can just ignore the fortified unit. It is just going to stand there, and if it actually attacks you, it is no longer fortified, meaning you can murder it. There is a reason why the AI very rarely uses fortification.