Mongol invasion of Rus'
Supposedly, Russian icons painted before the Mongol invasion of Rus' of 1237-1242 do not feature churches with onion domes. Two highly venerated pre-Mongol churches that have been rebuilt—the Assumption Cathedral and the Cathedral of Saint Demetrius, both in Vladimir—display golden helmet domes. Restoration work on several other ancient churches has revealed some fragments of former helmet-like domes below newer onion cupolas.