Trinity Cathedral is the oldest and most important church inside the Alexandrov Kremlin. Built in 1513, it keeps the stern proportions of early Muscovite architecture and feels much older than the later monastic buildings around it.
The details matter here. The cathedral preserves 16th-century frescoes and iron gates brought from Novgorod after Ivan the Terrible's campaign there in 1570, a grim trophy folded into a sacred building. It is one of those places where Russian history is not tidy, but it is very close.