It's abstracted with entrenchment, having landmines be a separate thing would be annoying and gimmicky to use like seamines.

Fort levels more than entrenchment as its not a static function, where entrenchment is building fieldworks ("digging in"). Though that's only valid if they're occupied. Both still requires defending troops presence for the effects to matter.

I think something where a province can have an "upgrade" that would give a modifier that caused enemy attrition increase in that province to properly simulate it better. I haven't seen it, but it would make for a nice mini-mod.