In another thread, I suggested that the attacker should have the option of attacking multiple targets at once. This was to allow a powerful unit (example tank) to kill off multiple horsemen or spearmen. The idea was that the combined defense values and hp would be applied to the attack. The attacker would have to decide without knowing for certain if there were 'hidden' defenders in the stack. Thus, there would be a command "attack multiple units" which would allow the attacker to specify how many units he wished to attack.
Should the attacker choose to attack 3 units, for example, then the best 3 defending units would be combined for hp and defense points. If there were 3 regular horsemen (for ex.) then the tank would face a 16 vs 3 and need to kill 9 hp. If, however, there were some mech infantry hiding in the stack...well, let's just say that 16 vs 72 where the 72 had 9 hp is not a very good attack. In addition, such an attack would prohibit retreat.
This, combined with the idea of doubling a/d pts for each era, would make it that keeping ancient units around to modern times would be both foolish and useless. Again, as an example, if a civ was defending a size 13 metropolis with 3 spearmen and I attacked with modern armor: I would have the option of attacking all 3 defenders (perhaps, since I do not know how many defenders there are, I would specify some quantity, let's say I chose 5). If I guess right and there are only spearmen defending the metropolis then they will combine their defense (to reflect the fact that I am going b@lls to the w@lls in attack) and have 6 defense. Since they are in a metropolis, this gets doubled to 12. Fortified gives an extra 25% for a grand total of 15. They are all regulars (in this example) so there are 9 hp of fighting to kill.
My force consists of a Modern Armor unit. This gives me a 24 attack. However, since I am 3 eras ahead of spearmen, I get 24*2*2*2, for a grand total of 192. A pretty good chance of wiping out these poor spearmen in one attack! I would then be allowed to use the Modern Armor unit for its other two moves (or if I had used up the moves getting there, the unit would take the city).
If, instead, I guess wrong and there is only 1 spearman and 3 infantry in the city (let's say the infantry are IA and have defense values of 12) then: the odds are now my 192 attack (with 3 hp) vs 2+(12*4) or 50 defense with 12 hp.
If disaster hits, and there are 5 mech inf. with 22 defense each: now I end up attacking at 24 (with 3 hp) versus a whopping 110 (with 15 hp!). So using this technique, I would be taking a big chance if my opponent hid some nasty defenders in his metropolis.
Thus, if a civ tries to defend with inferior units, the units are punished for their inferiority. The Civ2 technique of putting 100 spies in a walled city and having it defend against any comers is eliminated since you can attack multiple targets at once (which you could have a good idea of determining if you had already attacked the target and gotten a whimpy defender killed off). Someone trying to abuse the attack system could land his units in a fight from which there is no return.
One of the specifications for this type of attack is that the attacker can not retreat! This is to prevent the attacker from biting off more than he can chew just to find out the defense. It would simulate the idea that the attack was made as an all-out slam of the target.