It was fairly "game breaking" in real-life when it turned up as well.

Some really interesting fortification designs tried to mitigate the power of gunpowder. Earthwork backed walls, star-shaped fortress configurations, building underground instead of tall towers and castles.. all sorts of attempts were made to defend against gunpowder units. In the end, it spelled the demise of the classic static fortress "castle" as no walls alone could withstand any serious siege by gunpowder units. Fortresses became almost undergound, labyrinthine affairs with earthworks and fortified gun positions as walls were no longer enough to defend against men.
When stone failed, simple dirt prevailed. It was more suitable to absorbing shock and making a few holes in dirt with cannon didn't exactly debilitate the value of earthworks.. dirt and rubble just filled it back in. Advances in technology made projectiles capable of piercing through even the thickest stone and mortar configurations. Troops would wait out the barrage in fortified bunkers and come spilling out, taking to their fortified gun positions. If you can't kill the men, you can't take the position and even though you may have breached their defenses, you still had to overwhelm them. Not easy to do if they duck everytime you shoot. "Defilade" became the word of the day and demanded increasingly accurate and deadly mortar fire to be brought to bear to root out obscured targets immune to regular cannon. Being high on the walls of a castle overlooking the enemy was a sucker's bet when gunpowder came along.
All in all, not a pretty picture.