The castle you have is just a 'facade'. If you look at the source, you
will find that it defines an 'enter' command, but won't move the
player anywhere.

You have to make a hall way or something like that first. Do this by
copying one of the standard files (se ed4).

Now change the line in your castle.c that says

write("This is a close...");

into

call_other(this_player(), "move_player", "into castle#room/peter/hall.c");

The function "move_player" in the player object will move the player
to your hall, tell other players around that he goes into the castle.

See std for documentation about using std.h to make simple rooms.