IMO:
Under the 3e rules, your DM was wrong.
In real life, he was sort-of wrong. People did and do break walls with hammers.
The thing is, I don't think they can bust through a 5 foot thick stone wall, with one guy swinging one hammer, in less than half-an-hour. Even if the one guy does have the strength of two or three men. Worse, the hit points of a wall scale linearly -- so a 10 foot wall takes only twice as long -- that's less than an hour. For one guy -- one guy that's not using Power Attack.
There's a PC in my campaign that could use a warhammer to bash through an 8 foot thick wall in 30 minutes -- and that's using minimum damage, and a mere Medium-size hammer. Give him a good maul and use average damage, and he'll get through a 15 foot wall in 30 minutes.
Better, take a Str 16 guy with a Large hammer (2d6 damage; borrow one from an ogre or something). He does an average of 3 pts past the hardness of the wall every time he hits. THat's 300 hits to knock down a solid five foot thick stone wall (note that I'm talking about a "hewn stone wall" -- that's natural stone, not wimpy masonry). Assume he only swings twice a minute on average -- that's two-and-half hours to effectively tunnel through five feet of stone.
Hmm. One guy, working 40 hours a week, with two weeks' vacation, could hammer through a mile of solid stone -- using just his bare hands -- in 16 months. Add a couple of other guys to clear out the debris while he takes rest breaks (remember, he's averaging one swing every 30 seconds, so he's got time to rest), and you're doing pretty good, I'd think. John Henry'd be proud.
So what was up with all those sieges in the middle ages? Buncha idiots couldn't buy a couple of fairly strong guys (instead of one insanely strong guy) a half hour to knock a hole in those walls? What's with these long sieges, or using catapults or battering rams, or trying to scale walls, or having sappers dig all those tunnels? Idiots! Just knock down the walls with hammers.
