On 27 September 1808 , Napoleon was ceremonially presented the keys to the city at the Brühler-Tor before going to meet Tsar Alexander I on the road to Weimar to re-enter the city with the tsar . Between 27 September and 14 October 1808 , Napoleon hosted the Congress of Erfurt in the principality , intended to reaffirm the alliance with the Tsar , which had been concluded the previous year with the Treaties of Tilsit at the end of the War of the Fourth Coalition . The meeting became a great conference involving an array of kings , princes , dukes , barons and notables from all over Europe , including the kings of Saxony , Bavaria , Württemberg and Westphalia ( the last being Napoleon 's brother Jérôme ) . The resulting convention recognised the Russian conquests of Finland from Sweden and the Danubian Principalities from the Ottoman Empire and stated that , should France go to war again with Austria , Russia should make common cause , though the tsar 's support in the War of the Fifth Coalition was minimal
Congress of Erfurt
Congress of Erfurt
Principality of Erfurt