en silence -- in silence
silensieusement -- silently
Je monte les éscaliers en silence -- I go up the stairs in silence.
Je monte les éscaliers silencieusement -- I go silently up the stairs.
There is a slight difference in meaning. The first one means that you are not speaking while going up the stairs, whereas the second sentence means you take care not to make any noise. But if you always had to use "en silence", what use would the word "silencieusement" have? ;)
-- Chris (not a native speaker).