Indeed it is, and if a rogue in a party I'm running tries to backstab a character with a tower shield on their back, they're going to fail. The simple reason being that the shield isn't some nebulous image, it's a solid object, that is there. If their weapon wouldn't cut through it like butter from the front, it's not going to cut through it from behind either. That's a lot simpler than trying to explain, as I initially asked when this came up, where did that shield go?
Rogue backstabbing someone doesn't mean they literally stab the person in the back. It means they sneak up from behind and get a stealth attack. They catch their target off guard. It can mean jumping up on their back and slitting their throat. It can mean reaching around and knifing them in the side or in the chest. It can mean stabbing them in the back of the neck. It can mean bashing them in the back of the head.
See. One thing many people fail to realize is that D&D combat is meant to be general. It isn't meant to be specific.
I don't know if you've ever played Battletech, but THAT is a game where something like what you describe would count. Every time you hit, you roll to see where you hit and you do damage to that specific location.
D&D isn't like that. You hit, and you deal damage. The percentage of damage you do against the overall Current HP remaining simulates just how bad the hit was. A rogue sneaks up on someone from behind and hits them with a Sneak Attack, dealing 1d4+2 for dagger and 3d6 for sneak attack, they might deal 6 damage or 24 - or even more if a critical. Doing 6 damage would symbolize the character maybe striking the target at the wrong angle. The target realizes the rogue is there at the last second and tries to duck out of the way. The shield kinda gets in the way, being on the back, and the rogue maybe does a minor slit to the target's shoulder or something. A 24 or more might mean the rogue totally caught the target off guard and slit their throat or reached around and stabbed them in the heart. The rogue completely avoided the shield on the back and went for the weak spot.
