Hi leeneia,

Fels is one of the so-called "weak" masculine nouns - it's not archaic, there are a lot of very common nouns that add an n or en in declination, e.g. der Name, (accusative den Namen, dative dem Namen), and der Held (den Helden, dem Helden).

There's a good explanation with more examples here:

http://www.vistawide.com/german/grammar/german_nouns03.htm

John