Jim,
Here's what the designer has to say in the Campaign Notes:
<font color="beige">9. Building Breastworks:
Many scenarios in Campaign Peninsula will not allow breastwork construction in-game. While playing one of these scenarios, the player will see the message "Breastworks cannot be constructed in this scenario" when breastwork building is attempted. Another way to check this feature’s availability for a particular scenario is to view the Parameter Data Help menu—the building % will be 0. <b><font color="yellow">Historically, Civil War armies on the move and during battle did not generally construct makeshift breastworks at each opportunity during the war’s early years (1861-1862)</font id="yellow"></b>. Of course, there are notable exceptions to every generalization but, given that breastwork construction is unlimited within the game system, it would be more realistic to disallow battlefield breastwork construction altogether than to allow it to go unchecked at levels seen ordinarily only in the period from late-1863 through 1865. However, for those players that do not wish to be restricted, many of the historical and historical variant scenarios included in the game have versions that are different only in a player’s ability to build breastworks.</font id="beige">
[url="http://homepage.ntlworld.com/a.r.barlow/acwgc/acw.htm"]General Antony Barlow[/url]
[url="http://homepage.ntlworld.com/a.r.barlow/acwgc/western_theater.htm"]Commander, Western Theater, Union Army[/url]