/shrug. And I think you're being a bit narrow in your interpretation. You're trying to classify things based on what they physically are, rather than how they may be used. Every offense can be used defensively, and every defense can be used offensively. A wall doesn't defend anything unless there are people manning it. They man it by "attacking" those who are trying to get through/over/under the wall. One can easily argue that a defensive position increases the number of opponents you can kill by a factor far greater than any "offensive weapon" will.
The Walls are defensive because they allow the men on them to attack without being attacked. Walls directly defend the people they mean to protect. Always have. Before siege weaponry Walls defended cities without even needing men, because they were impossible to counter...until offensive weaponry allowed for others to destroy those walls.
Armor is defensive and it revolutionized combat. It made stone weapons obsolete, it forced crasftmen to forge bronze and iron weapons so people could effectively kill others. Heavy Armor required men to devise a way to assault from long range, so the Long Bow was devised to rain destruction down on armor clad knights from a great distance.
The gun was developed to kill people en masse from great range. The gun evolved over the years with one objective, to be capable of eliminating as many people as possible. Gattling Guns, Tommy Guns, AKs. Guns designed for one purpose...killing people as quickly as possible.
The gun is 100% an offensive weapon. At best it provides intimidation to negate a negative act. Then again Intimidation is also an offensive gesture. A gun does not protect you from anything...it is a security blanket mentality, guns are designed for one purpose. Killing period.