It's been a while since I looked at the game, but I'm pretty sure a trench is just a unit with 0 movement that has a high defensive value. Just like in regular civ 2, a unit can't prevent stack kills or increase the defence of other units, but the scenario designer probably put fortresses on the squares where he placed trenches.
If the trench is not listed in the Civlopedia, then it will have a prerequisite of NON, so it will be impossible to build more, because there is no technology that enables it. Check your defence minister for the attack/defence value of trenches.