Editors. "A French army forces the imperialists to raise the siege of Heidelberg". The Literary Encyclopedia. First published 01 February 2011
[http://www.litencyc.com/php/stopics.php?rec=true&UID=16083, accessed 18 April 2014.]
Heidelberg is being besieged by the imperialists, but the arrival of a French army in the city's defence forces them to raise the siege.