"It's somewhere in between, not as big as a whole temple, but the main problem is that there is also what looks like the ruins of a shrine complex around it--nothing fancy, probably just residence for the keepers of the shrine, stables, a place to grow food, and the like. But the elves have guarded the perimeter of the whole complex, and the secret tunnel goes for a long ways before it opens into the underground room."
"Actually, I didn't know exactly how far Dimension Door could go. So--"
"Those shoes nigh-undoubtably will reach 680 feet. If it would making the difference, having one of the three of us cast the spell instead could increase it to 760."
"I think that could actually be enough if you could avoid the elves enough to get right up next to the shrine proper--I think it's about 200 feet from there to the centre of the shrine, perhaps a bit more, and then over 700 feet straight down from there to the secret chamber."
"So call it 210 feet and 720 feet. The square root of 210 squared + 720 squared is 750. That's in range for the casters, but not the shoes."