An Artificer's Guide to Siege Weapons

 So, I love the Artificer. I love the concept, the class features, the flavor, and the overall execution. Yes, I realize I should probably play a pure caster that isn't a Warlock at some point (it'd be nice to cast high-level spells one day,) but I'm extremely drawn to the artificer.

I did some table-napkin math today and came up with something awesome that an artificer could do if they needed to besiege a castle.

Ok, couple prerequisites:

You need a party with at least five friends in addition to the artificer.

You need a catapult.

You need a 20-foot block of granite.

And you need to be level 11 or higher.

Artificers get a potentially game-breakingly good ability at level 11: spell storing item. You can imbue an item with a single spell of 1st or 2nd level that takes on action to cast. The item has charges equal to twice your intelligence modifier (likely 10 by level 11,) and while it uses your spellcasting ability for its attack bonus or save DC, anyone can use the object, and that person maintains concentration if the spell requires it.

Now, you make one of these things with enlarge/reduce in it.

Ok, you've used some means to bring this massive cube to the battlefield - maybe your wizard teleported it there or something. (Teleport requires an object fit in a 10-ft cube, but we already have enlarge/reduce, don't we?)

Now, you set up your catapult to launch its payload at the enemy castle. You and your five friends cast the reduce side of enlarge/reduce on the cube of granite, reducing it from gargantuan to huge, huge to large, large to medium, medium to small, and small to tiny. The cube's weight is reduced to 1/8th of its normal weight each time it's reduced, so after five castings, you've reduced this thing to .003% of its usual weight. Given the density of granite, that makes it about 7.5 inches on a side, and it weights about 41 pounds. That's not light, exactly, but it's well within the normal payload of a catapult.

You don't have long, but you get this cube into the catapult and then let 'er rip. While the thing is flying in mid-air, everyone drops concentration, and this 41 lbs cube expands into a 686-ton castle-destroyer.

Now, what about momentum? The good news is that as long as you know how momentum works with that spell (the sort of thing the DM would rule) you can make it hit the target either way.

If velocity is maintained regardless of mass, then you just aim it at the castle and smash it with its far-higher-than-it-should be momentum - essentially, the momentum grows by a factor of 33,000.

If, however, it's momentum that is maintained but not velocity, that means that as soon as the spell ends when your party drops concentration, the block will more or less come to a stop (technically its speed will drop to .00003 of its previous speed, which will look a whole lot like it just stopped.)

If this is the case, no worries. Instead, just aim above the castle (high up, if you can.) Then, as soon as it's right above the target, you drop concentration and this 20-ft cube of granite will plummet directly on top of the castle.

As long as you know ahead of time how that works (and a good artificer will experiment to understand the spell before attempting something like this,) you'll be prepared to use it correctly.

And now, any object that isn't made of adamantium is going to be utterly destroyed by your brick from the sky.

And the best part?

You can do this twice in a single day.