Just a little one this, Pete Ball, one fo my regular commenters and reader has been exploring his terrain building efforts. He's had loads of simple CD sized ideas on his own blog recently - earthworks, watchtowers and the rather effective bunkers below:

http://veganhaggis.blogspot.co.uk/2013/02/terrain-tutorial-simpler-mini-bunkers.html

Check out his blog on the link above and this compendium of the recent articles to see if there's something that would suit your battlefields.