I immedietly thought "turtler that hates rushers" when I read your post.
Your suggestion makes early attacks less valuable by a great deal and simultaneously makes turtlers (people who don't want to spend a cent on defense) stronger. This can't be allowed to happen.
Minimum battlerating to attack a town (empty or not) also weakens smaller armies and rewards stack of doom and turtlers.
Don't ad hominid me by interpreting me as an "inferior" player based off a forum post about a game that isn't out yet. Also, turtles DO spend a cent on defense. In fact, the primary characteristic of a turtle is that they buy no OFFENSE. Only defenses (like turrets or forts or whatever) and economy buildings. Pure eco "boomers" are the ones who tend to purchase no defense, and will do a small offensive harass to preoccupy the opponents while they build up their econ.
I actually play rush strategies.
Early game, cities wouldn't have a population of more than 50, at most. They start at 10, correct? This mechanic might trigger, but it won't be USEFUL. One peasant isn't going to help against a dedicated army. The minimum battle rating isn't to disable attacking. It's if you don't meet that threshold, you meet resistance. Dropping into a tactical battle. Which would enable both sides to use spells... like summon an Elemental or use a Fireball. Both the attacker and the defender.
We also currently have no mechanics that award defending (that I know of, at least). This is one. It should be more difficult to attack than defend... that is nothing new, really.
Civ V is doing this as well (cities defend themselves).
Civ IV: Colonization is also similar. There is a resource called "guns" that exists. If you have a city that is attacked for every 25 guns one of your civilians will form a militia to protect themselves (you can have "soldier class" civilians as well who have a higher battle rating). Finally, you will have one unarmed peasant defend the city after your gun-wielding civilians have been defeated. That game is different than other Civs because, in fact, it's BAD to have a "standing army" garrisoned as a city. If you aren't planning on attacking someone soon, you should always settle your soldiers into a city for a few turns.
So... this isn't something that doesn't ever happen in other games?
I also only pointed this out to "mundane" attackers. An enemy sovereign or a high-level champion or a fantastic monster would be able to conquer an enemy without this punishment. Which is what a rusher would be doing. So, an army purely composed of low level "thugs" without a leader would have difficulties just occupying a city, even if they destroyed their weaker defenses. Similarly, if there is a very close battle but you (as a defender) manage to kill the enemy leader and a large portion of the attackers, the civilians are "rallied" into defending their homes.
However, Brad mentioned that animals and bandits will start being aggressive now. Peasants defending themselves (and their wife and children) MAKES SENSE in this context. These people have been wandering the wilderness for 100 years without a home. they finally have one now, and they have learned how to defend themselves over the years.