I suppose Petersburg would be a difficult design primarily because two types of engagements that might be fought. On the one hand, at engagements like Jerusalem Plank Road and Five Forks, units of both sides came out into the open in regular formations. On the other hand, when/if Grant decides to hit the main reb line at the Crater or in 1865, the Rebel infantry is likely to be spread out in companies and battalions to cover the trench lines. Even at Fort Stedman Union forces might have been broken down into companies to cover the union trench lines. There might therefore be only a couple of ways to deal with the situation: 1) Have Confederate formations split to smaller sizes such as companies (lots of counter pushing) 2) Allow only hard zones of control in trench assault scenarios (if the bigger units can cover all the ground) 3) Allow units to 'breakdown' and 'buildup' which can be done in the PZ Camp. series 4) Simply skip any options for Grant to assault the main Petersburg line meaning there would be no need to have smaller units (though the point of a campaign would be to lead to the final assault or some attempt by the rebs to flee or launch their own attack)... All that being said, I still think it is a worth title. A new book is also coming out on the subject by none other than Edwin Bearss which might shed some much needed fresh light on the campaign. It looks like a good read...
http://www.amazon.com/PETERSBURG-CAMPAIGN-Eastern-Battles-August/dp/1611210909/ref=sr_1_3?s=books&ie=UTF8&qid=1329020456&sr=1-3One quick note on the cumulative defensive modifier problem when dealing with advanced works. Hypothetically speaking, if we allowed units to build 3 levels of works with a 70% modifier for the highest level, an advanced fortification in a forest would result in a defensive bonus over 100%. I suppose the easiest way to deal with the problem would be to not allow the most advanced works in trees or rough terrain. From the pictures I've seen, most advances works were in fact in the open with clear fields of fire. Thus it might be workable to allow the most advanced fortification in a clear hex, but not in a wooded one. After all, digging deep entrenchments and embankments in heavy forest just ain't going to happen without clear the trees, roots and rocks. It would be much easier to do it in a Farmer's field in because the ground has been regularly tilled.