As f" solved for Saturn
and Cain reasoned Stairs
While one word you seek
Three this riddle bears:
Carved from a forest we lie
A road built of bone we supply
With great walls of black
We are known to attack
From the forest an echo replies
For a singular purpose we're made
To deny those of whom you're afraid
And provide peace of mind
Each day we're combined
With our partner who therein has stayed
At prominence ere the fall of Rome
We held the theaters, bridges and domes
In structures erected
We shan't be neglected
Lest one desires to topple our home
The first stanza is describing the method with which this riddle was written. For instance, I thought of many things which referred to Saturn and described them. Though technically separate words (homonyms, in fact), the answer is just one.
By now you may have noticed that the second stanza is buried deep in metaphor. There is no real forest. There is no real road. There are no walls. They do attack, but not in a violent sense. The forest does respond to the attack with a sound, however.
A biologist may cringe at my use of the word "bone" in the second stanza. It shouldn't prevent anyone from solving it, but it is scientifically inaccurate, so I apologize to those feeling scrupulous.