There seems to be some question about Aachen. See the discussion at 
_http://medievalnews.blogspot.com/2010/05/grave-of-charlemagne-remains-mystery.html_
 
(http://medievalnews.blogspot.com/2010/05/grave-of-charlemagne-remains-mystery.html) 
 
 
In a message dated 12/28/2011 10:35:27 A.M. Pacific Standard Time,  
[log in to unmask] writes:

Chapter  XV of The Innocents Abroad has several interesting points
including his  interpretation of the story of Abelard and Heloise.  He
visited two  burial grounds, Pere la Chaise and St. Denis.  Apparently he
found  Charlemagne at St. Denis.  I looked up Charlemagne in Wikipedia
and it  has him interred in a vault in Aachen Cathedral, in modern day
Germany.  This chapter of Innocents is not represented in any of the Alta
letters so  must be a product of recollection while writing the book. If
this is indeed  an error on Twain's part, how could this have happened?