There are maps where you’ll encounter obstacles that will hinder your progress until you perform specific actions like defeating certain enemies or destroying certain objects. These barriers are meant to lock you out early in the battle while you work your way in completing the other objectives first.
One example are sandstorms and fogs. While it isn’t necessarily a barrier since you can plow inside a blowing sandstorm and still see the enemies inside, it still hinders vital functions like having the ability to see the details of both enemy and allied units inside, as well as disabling the ability to issue orders to your allies, and enemies inside can’t be selected as targets via command orders as well.
Darkness walls are put up by enemy mages in order to protect a particular fort and prevent access to it. To destroy this wall, you have to hunt down the mage(s) that activated it. More powerful walls will usually have a wall that’s controlled by 2 or more mages. Defeating them will remove the wall, allowing your forces to pour into that enemy stronghold.
Ledges are not intended to be obstacles but they can considered as such since they’ll only allow one-way entry to a particular area due to elevation difference.
Lava is a type of terrain that you’ll encounter in certain Nohrian maps. Ground units standing on it will take small but continuous damage while flying units are immune from this damage. Lava can be cooled off by capturing a fort with a Dragon Vein inside, then activating that vein to change the terrain to a harmless, solid ground.
Flight Paths are gaps in the Map that can only be traversed by flying units such as Pegasus Knights and Wyvern Riders. These serve as valuable shortcuts that can be used by allied and enemy forces alike. Some flight paths like the ones in the World Tree map can become solid bridges after activating their corresponding dragon veins.