As a rule troops of this era were not equipped to construct field fortifications. However, if given time and the site of the battlefield was know in advance they could be issued with spades etc. and required to construct breastworks and redoubts etc. The Battle of Borodino is probably a good example where the Russian's had put a lot of effort into preparing the gorund before the arrival of the French.

This would suggest something more closely akin to the 'Build Fort' option on the strategic map. Perhaps, awarding a fortification point everytime its clicked which can then be spent during deployment on various fieldworks, a bit like the construction system for seige engines.