It is unlikely that the Germans named the lake so for being shallow since the adjective platt is a Greek loanword that was borrowed via French and entered the general German vocabulary in the 17th century. The German name for the lake is Plattensee. Paleolinguists surmise that 'Pelso' meant 'shallow' in Illyrian this deduction is based on a surmised Proto-Indo-European root *pels. Pelso derives from a local name for the lake, perhaps from the Illyrian language, as the Illyrians once populated the region. The Romans called the lake Lacus Pelso ('Lake Pelso'). His extremely well fortified castle and capital of Balaton Principality that became known as Blatnohrad or Moosburg ('Swamp Fortress') served as a bulwark both against the Bulgarians and the Moravians.
Slavic prince Pribina began to build in January 846 a large fortress as his seat of power and several churches in the region of Lake Balaton, in a territory of modern Zalavár surrounded by forests and swamps along the river Zala. This name derives from the Slavic blato meaning 'mud' or 'swamp' (from earlier Proto-Slavic boltьno, Slovene: Blatno jezero, Slovak: Blatenské jazero ). In Hungarian, the lake is known simply as a Balaton, or 'the Balaton'.
