Magical trench warfare: Asking for sugestions

So, I have this world that was perfectly at peace, until the party came around. Now, It's basically MWW (magical world war). And my players are looking foward to some trench warfare in dnd.

Why trench warfare? Because mold earth is a cantrip and it is always better to have cover. I have a couple of basic ideas.

The bullet points are:

Scrolls, rare magic items and more than lv5 npcs are hard to come by, because they are strategical game changers froom both sides.

There is gunpowder in the world and the spell detect traps actually detects traps (location of all in an area), so, landmines are a thing.

Someone as figured out ballista machine guns, so charging is unadvised.

I'm looking for tactics and general suggestions.