A wall with a firing port to allow shooting from relative safety.
About embrasures Edit
Embrasures are extremely useful if you're making pillboxes or killboxes. As embrasures provide more cover than sandbags and enemies cannot walk through them.
Do keep in mind that enemies can also fire through these embrasures from their side. Because of this, you should watch out for tribal boomers or pirate grenadiers. As they can quickly throw their explosives through the firing port and cause grievous injuries or even death.
Cheesing with embrasures Edit
Because of the fact that they act like walls, pawns obviously cannot walk through them. Because of this you can cheese early tribal raids, early pirate raids and manhunter pack events.
As melee fighters can't do much with embrasures (Unless they are sappers) except look through them and (unmodded) animals can only attack in melee. Which gives your pawns armed with firearms or bows behind them a nice bit of target practice. This is also why some people look down upon using the embrasure. As the melee users cannot retaliate against the shooters behind the embrasure.
Things to note when using embrasures Edit
You should gives your pawns helmets if you're fighting against later pirate and tribal raids. As only the head is visible for the enemy and because of that most hits will result in a headshot. Which can get nasty really fast if not wearing a helmet.
Also make sure that your embrasures are made from non-flammable materials. (so avoid wood, steel, gold, plasteel and silver)