What do noun clauses begin with and what do they function as?

A clause has a subject and a verb and functions as one part of speech. (It follows therefore that a noun clause functions as a noun in a sentence.) Lots of noun clauses start with "that," "how," or a "wh"-word (i.e., "what," "who," "which," "when," "where," or "why").

Explanation:

1.the hosts on two opposing mountains stood,  

2.thick as the foliage of the waving wood;  

3.between them an extensive valley lay,  

4.o'er which the gleaming armour pour'd the day,  

5.when from the camp of the philistine foes,

6. dreadful to view, a mighty warrior rose; in the dire deeds of bleeding battle skill'd,