I am the hidden land
Tolkien occasionally referred to Lothlorien as 'the hidden land'
Where man may not tread but wildlife can
Man, Dwarves, orc etc. are forbidden from entering Lothlorien without the permission of the elves
Here a group in full did travel
By peak twilight, by horse and paddle
If we look at the history of lothlorien, it was initially settled by Silvan elves of Nandorin descent. Some time later the Sindar elves arrived and enriched the population and culture of Lothlorien
The group spoke sin and thus went in
This was a play on words, the Sindar elves spoke Sindarin and entered into Lothlorien
To rule over all within
The Sindar elves enriched lothlorien with Sindar culture, their ruler was the Sindar lord Amdír.
References to descriptions given above can be found here.
Where am I?
HINT 1
Man may not tread in this place but man is not the only species in this universe
HINT 2
This place is a place of great beauty and serenity
HINT 3
Man may only enter this place if invited in
Final Hint
A fellowship also visit this place at some point in their quest.