A sacred grove or sacred woods are any grove of trees that are of special religious importance to a particular culture, during the Northern Crusades, were religious wars undertaken by Catholic Christian military orders and kingdoms, there was a common practice of building churches on the sites of what?
Sacred grove:  During the Northern Crusades, there was a common practice of building churches on the sites of sacred groves. Northern Crusades: The Northern Crusades or Baltic Crusades were religious wars undertaken by Catholic Christian military orders and kingdoms, primarily against the pagan Baltic, Finnic and Western Slavic peoples around the southern and eastern shores of the Baltic Sea, and to a lesser extent also against Orthodox Christian Slavs (East Slavs).
sacred groves