Oh God no, my mind just exploded. That would be too much. I don’t know how much time I spent browsing “Click and Drag.”
(Hidden amongst all the urls is the GPS location of the actual sandcastle that Randall has already built!) 
Thank you for this thread. I’d been off on vacation for a few days and when I clicked through XKCD I saw a guy with a sandcastle and the alt-text of “wait for it.”
So I did. I kept it loaded all day on another tab. Nothing happened.
So, the couple just went for a swim, and it looks like the very back end of the sandcastle eroded a bit…
I’m taking advantage of my medium’s impermanence… This sculpture is about transience. As this figure erodes, it invites the viewer to contemplate the evanescence of life. This piece speaks to the horror of our own mortality!
Yes, you’re right. A corner seemed to collapse. She just came back from the swim coughing, but said she just “got some in her mouth”. Presumably she means water.
You are? Are you Randall then?
Like sands through a sandcastle, so are the days of our lives.
I’ve only just spotted the music note in the lower left corner. Click for added ambience.
I think that was a quote from a Calvin and Hobbes snowman-sculpture strip.
He’s bridging the gap…
What do it mean?! Maybe they’re playing real-life Minecraft.
Man, I hate it when I miss a reference. That’s it; from now on I google everything.
And it now looks like leahcim was right about the dueling trebuchets. At least, there appears to have been some projectile that arced in and damaged the castle.
Yeah, the woman is now on the far left with a trebuchet aimed further to the left at someone or something else.
I wonder how long this is going to run?
It’s aimed right.
Huh. So it is. It threw me since that means she is sitting in front of it.
And in true psychic fashion, I intend to tout my prediction of trebuchets as proof of my l33t powers of coolness, and not mention my many other wrong guesses ever again. 
(Although if it was procedurally generated, and those procedures included randomized trebuchet attacks, that would be fifty shades of awesome.)
I’ll go out on a limb and predict that it’s going to be far longer than we suspect right now. Like, probably months, maybe years.
Like I said, 97 minute film animated at 4fps and reeled out over two years.
Speaking for myself and all the rest of us who do webcomics?
Randall, stop making us all look bad, Mmmkay?
They haven’t quite got the hang of that trebuchet, have they.
Vertical throw, almost hit themselves.
And now, reconstruction.