I'll provide a more detailed explanation of how the game will work, but twin sticks definitely will not be on the cards.
Think; tower defense/top down shooter/base building/wave orientated.
Very fitting for the first world war. The whole thing was a very complicated mexican standoff. Units would occasionally charge over the trench walls in a mad dash across the field to get into the opposing area. I would recommend against making it a pure defense game and add some aggressive elements to it.
While the enemy can send waves at you, you can also send waves at them. The units would be very easily killed of course and get pinned down or killed in the no-man's-land. You could deploy different tactics such as investing an in-game currency to spend on units or research technologies. You could also have units work on making a tunnel to the other side and rigging explosives. Or you could even invest heavily in armor technologies and send tanks rolling across their trenches, (their obvious weakness being artillery or even breakdowns, since WW1 tanks were not very reliable).
machine gunners could have blind spots
tanks could have breakdowns
infantry in no-mans-land would be weak to entrenched infantry
artillery would have a long cooldown time etc
The actual no-man's-land could be a place where dead infantry and failed charges "pile" up, making it more difficult to cross due to barb wire and bodies as well.
