As referee I would allow a mobile target like a wagon or saddle provided 1) the caster was very familiar with the target in question and 2) the caster knew generally where it was (i.e., within a few miles).

If the caster had no idea at all where the wagon or saddle was, the spell would fail.

DMPrata wrote:

I would say no to the wagon or saddle ("...the destination must be an unoccupied location on solid ground"), but perhaps yes to the floating citadel, if sufficiently massive to encompass "solid ground". The caster's degree of familiarity would depend upon the citadel's current location, however.

Yeah. Given the consequences for messing up, this is just not a spell whose limits I would try to push, were I a magician (*maybe* if I were a pyromancer; they'll tear your lungs out, Jim!).

gizmomathboy wrote:

Well, I think how much warning/guidance I would give about the limits would depend upon my mood I think.

I'm more of the "well, you can try it and see what happens" mindset usually.

Sure--the consequences are spelled out right there in the Players Manual.
 

And some rather interesting answers. Like, "It's dark. You might be inside your horse."

DMPrata wrote:

Handy Haversack wrote:

And some rather interesting answers. Like, "It's dark. You might be inside your horse."

 "...and I thought they smelled bad on the outside!"

Exactly! Haha!