This vase is a beauty and extremely rare. It is also very skillfully decorated, from the scenic panel (known as a "veduta" or 'topographical') in colors at the front, to the densely decorated rinceaux and lambrequin patterns in gold enamel, all meticulously painted by hand.
The form of the urn itself is derived from a classical Hellenic Greek vessel called a krater (in German known as a Redensche vase). All of these can be traced back to one in particular called the Medici krater, now in the Uffizi gallery in Florence, over five foot tall, and carved from a single block of marble by the Greeks nearly 2000 years ago and sold to the Romans as a garden ornament.
So what about the date of yours? This is certainly a very old piece of KPM, and from way before 1837 when the initials "KPM" became part of the mark. The form and the type of decoration, and the sceptre mark on the bottom, all date it emphatically to the German Neo-Classicism period, circa 1790 - 1810.
So this is a museum piece. KPM veduta Medici vases from this period do not come up for sale that often, as you may imagine, so finding comparables is not easy.
However, if you were ever to sell, and I would recommend a top antique auction house such as Christies, Bonhams, Skinners, etc, it would fetch in the range of $5000 - $7500. Possibly more since you have its entire provenance and history since at least as far back as the 1830s. If you can add any information as to its link to Wartburg Castle, then you can certainly nudge the upper end of the auction value up to $8K or more.
I would therefore insure it for at least $15,000.
I do hope this helps!
Please let me know if you'd like me to explain or expand on any of the above, I would be glad to.