This is the 10518th most frequent Russian word.
виднелись
"Appeared" or "were visible."
The word 'виднелись' is used to describe something coming into view or being able to be seen, often faintly or from a distance. Here, it conveys the visibility of mountains far away.
Вдали виднелись заснеженные вершины гор.
Snow-covered mountain peaks were visible in the distance.
In this context, 'виднелись' indicates the appearance of the castle's silhouette at a distance against the horizon.
На горизонте виднелись очертания старого замка.
The outline of an old castle was visible on the horizon.
Here, 'виднелись' communicates the notion of visibility through an obstructive mist, emphasizing faint perception.