The battle was fought on the lake during the winter when it was frozen over.

Battle of the Ice

The Battle on the Ice (Russian: Ледовое побоище, Ledovoye poboish'ye; German: Schlacht auf dem Eise; Estonian: Jäälahing; Latvian: Ledus kauja), also known as the Battle of Lake Peipus (German: Schlacht auf dem Peipussee; Russian: битва на Чудском озере, bitva na Chudskom ozere), was a battle between the Republic of Novgorod and the Livonian branch of the Teutonic Knights (whose army consisted mostly of Estonians) on April 5, 1242, at Lake Peipus. The battle is notable for having been fought largely on the frozen lake.

• http://en.wikipedia.org/wiki/Battle of the Ice
• Spilling, M., 2010, Battles that Changed History: Key Battles that Decided the Fate of Nations: Amber Books, London, 448p