The Church of the Nativity of the Theotokos "na Senyakh" is the oldest surviving building in the Moscow Kremlin and the city of Moscow. It has been part of the Grand Kremlin Palace since the mid-19th century. The church cannot be visited. Only the underground floor of the Annunciation Cathedral goes back to the same period of the Middle Ages as this church.
The church was built in 1393 or 1394 by Princess Eudoxia of Moscow, the widow of Dmitry Donskoy. It adjoined the hall of the royal palace, hence its name "at the hall".
Wikipedia https://en.wikipedia.org/wiki/Nativity_Church_in_the_Kremlin
Address (Unnamed Road), Москва 109012, Russia
Coordinates 55°45'1.954" N 37°36'53.888" E