Delicious but an absolute fail on my part

I live at 5,200 feet. Yes, altitude makes a huge difference in baking.

I strongly recommend getting a recipe developed and tested at high altitude, and then not deviating in any significant way. (Which pan you use seems to be one of the most important factors in success vs. failure when baking cakes at altitude.)

I highly recommend Susan Purdy's excellent, "Pie in the Sky" - not just about pie, it has thoroughly tested recipes for cakes, cookies, pies, bars, yeast breads, etc., - with variations depending on your actual altitude. What one must do here at 5,200 is different from what you must do to make it work at 7,000 feet.

Good luck! ;o)

Yes, the loaf pan choice was the source of your problems. Assuming you used a standard 9x5 loaf pan, it only has a capacity for a maximum of 8 cups of batter. A 10 inch Bundt pan holds 10-12 cups of batter. If you put all that batter in a loaf pan, a lot of it would have to tumble out in the baking because it had no place else to go. Some cake batters are also so dense they really do require a central open spot to ensure heat gets evenly distributed during baking, so center and outside gets done at roughly the same time. A loaf pan obviously lacks that, so your cake slumped because the center was not done enough to hold structure in cooling. And yes, high altitude plays havoc with baking, and you would need to adjust the amount of leavening, the baking powder, to allow for this. I'm not adept at altitude baking, but I recommend checking out sites like King Arthur or your local agriculture department for advice on that. And I recommend you get a cake with a center- either a bundt pan or an angel food pan, to rebake this cake in. Then you should have better luck.