Nael2009 wrote:

I would like sth like be able to build a trench. 

If you get the technology for trench warfare you could build trenches as a unit action.

Here are my ideas for the trenches.

1.  It works like a river to climb in the trench and go out of the trench. 
2. You can shoot out of a trench.
3. Your troops inside a trench get a lot less damage by artillery and a bit less by Bombardments by planes.
4. When your troops inside a trench get attacked by a ranged unit (like the Red coats) they get no-a bit of damage
5.  How long it takes to finish the trench depends on the Combat strength and the industry value of the district.
6. Trenches can be built in cities but for each trench, half the value of the respective district is deducted.
7. Trenches can also be built-in battles. The trenches will stay after the battle.
8. One Trench is one district big and will combine with one around him.
9. Built trenches stay in Battle.

I would like this feature because it would do a very good job of expanding warfare and making it more realistic

There is a unit effect "Dug in" which applies automatically if the unit is stationary

Now I never actually made it till that unit so far before the game ended but might be what you are looking for: it should have similar effect and maybe even graphical detaisl to signal it on the tactical map

I am fully with you in wanting to see WW1 (also WW2) frontlines and such a mechanic should enable that if the AI knows how to make user of it and react to it - and it woudl likely be best if it was an automatic effect

I dont know if it would be good to allow building them beforehand or leavign them around even after the batle - Civ V has tried something like this if I remember correctly and it didnt realyl work well there due to the macro involved (but also likely because the AI was utterly incapable of utilizing or react to it)