Sloping castle walls were probably the best defense against cannon (round walls protected against undermining). You're right about getting enough of a reaction out of the powder - it was because of that that high walls became obsolete altogether.
But round shot was still used all the way into the 19th century. Earth works and bastions with brick structures were the prime method of defense in those times. I don't know what part cone-shaped bullets actually played - I think that was just the invention of cartridges and shells, which was more of a matter of loading convenience.
"But round shot was still used all the way into the 19th century."
Of course. The replacement by cone-shaped projectiles didn't happened in one month or one decade only.
"I don't know what part cone-shaped bullets actually played - I think that was just the invention of cartridges and shells, which was more of a matter of loading convenience."
Well, I'm no expert, but if one digs in a bit, one will find that cone-shaped projectile have a much better penetration, whence destructive power, especially if you add time-fuse exploding shells (doesn't explode on impact), and rifled bore.
It was not only easier and faster to reload, but a cone-shaped bullet, together with a metal case, made it possible to fire even under the heaviest of rains.
The part they played? Hmmm ... well, in France, in the aftermath of the unfortunate War of 1870-1871, which saw us loosing a part of our eastern metropolitan territory, it was decided to build an ensemble of forts, all along our border with Germany (we still called Prussia, at that time) and also some coasts, according to wikipedia. This great task was supervised, and initiated, by brigadier general Raymond Adolphe Séré de Rivières, who gave his family name to the system of fortifications being built.
Now, the thing is, as I stated, that these fortifications were being built in the aftermath of France-Prussia War, during which the cannons fired good old rounded balls. So the plans were made according to this, and so the forts of the Séré de Rivières' system were all built in masonry and an good layer of earth.
But, of course, building tens and tens of fortifications requires not only a huge amount of men, materials, and money, but also, obviously, a huge amount of time. And in 1880, the shell then known as "torpedo shell" - or cone-shaped, as we said - was invented. And in 1884, the smokeless powder is invented in France. And it is much more powerful than the black powder. Of course, this smokeless powder will see others similar products invented and improved elsewhere in Europe.
The Séré de Rivières System was finished in 1885. So by the time the torpedo shell and smokeless explosives were put in use in military ordnance, it was too late to drastically modify what was almost finished being built.
Oh, of course, it had been decided, after 1885, to reinforce the already obsolete Séré de Rivières' forts with military-grade concrete, but this required also a considerable amount of time, men, and money. And the task wasn't completed before 1914.
Some forts were simply blown up, at least partially, by the new projectiles. And the garrisons, seeing that most of their bricks-and-cement fort were of little to no use to protect them, after hellish shelling, had no other choice than surrender. Continuing resistance would have, in certain cases, only added more casualties on French side, with no consequence on the German side.
That might be a part of the part played by cone-shaped projectiles in artillery.
Well, the new, smokeless ballistic explosives were also much, much, MUCH more powerful than the black powder for the same weigh.
1) How does it affect the ability to remove the fortresses' walls faster? By arriving on it much faster, of course. The maximum muzzle speed attainable by a black powder propelled round cannon ball is at a certain level. It could reach super sonic speeds, yes (saw that in a "Lock N' Load" TV show, by R. Lee Ermey), but it wasn't that much above the sound speed. The new generation of bullets and ammo, however, were almost always super sonic (maybe except the handguns' ammo) and much higher above the sound speed than the black powder bullets.
The bottom line of this is that a cannon shell from the "new generation" delivers a much greater energy upon impact.
2) In addition, there is the shape of the projectile. A rounded shell is more likely to bounce off, or to be stopped and fall harmlessly down. A torpedo-shell, though, for obvious physical reasons penetrates more easily thanks to its form. You might manage to put a flat-point nail through a wall of, say, cement, but it will be much easier if, with the same amount of force, you use a sharp-point nail (a common pointy nail).
The bottom line here is that a conical shell penetrates more easily. It makes holes more easily, so it's a better and faster destroyer.
And if, on top of that, you add the force of the explosion made by a shell filled not with black powder, but with newer explosives, more or less akin to those propelling it out of the gun, you have the explanation why older, masonry forts were completely useless against them.
"you mean explosives got better"
Of course, they did.
"but that has nothing to do with conical bullets"
See 2). Plus, even if there were already explosive round shells, filled with black powder and shrapnels, it wouldn't be that more useful against the masonry forts. If the shell penetrates the masonry wall before exploding, the effect will be better than if it explodes against the wall without penetrating it.
P.S.: I'm not a professionnal of that kind of things. At most, I'm a amateur who wishes to read trusty sources. For better explanations, ask a professionnal historian specialist of that things.