One of the ways of proving historicity today is to show archeological proof or evidence. But continuous civilization, while a marvel by itself, does not leave intact, sufficient archeological remnants of its ancestors. Generations come and go and as they keep building and rebuilding their cities, they destroy or alter these remains in order to sustain themselves. Hence while the continuous civilizations and culture itself is an existing evidence, archeological artifacts are hard to come by.
To understand this, think of a clock which stopped ticking during an earthquake. If nobody repaired it, one can state with assertion the time of the quake. But if it were to continue ticking, then the clock ceases to be an evidence. Today, small occassional finds in deserted places are all what we have, to attempt to learn about our history.
It is because the deep seas swallowed Dwaraka, the town became uninhabitable and frozen in time and hence after so many thousands of years, it is still able to provide evidence for the existence of an advanced township tallying with the city described in the epic Mahabharata.
The only such uninhabited place to look for evidences around Rama’s times as per the epic Valmiki Ramayana, would be to study the Rama Setu or Adam’s Bridge.
Since all traditional archealogical evidences are not directly available due to an elapse of time of more than 7,200 years coupled continuous civilization, the bridge is probably the only archaeological evidence!
Further, in the Mahabartaha text, when the 5 Pandava brothers roam the forest during their exile period, they visit the Ashrama of Rishi Markendeya, who regales them with the histories of their land. One such story is of the Nala Setu, the text of which is given below.
...And that mighty army, protected by Nala and Nila and Angada and Kratha and Mainda and Dwivida, marched forth for achieving the purpose of Raghava. And encamping successively, without interruption of any kind, on wide and healthy tracts and valleys abounding with fruits and roots and water and honey and meat, the vanara at last reached the shores of the briny sea. And like unto a second ocean, that mighty army with its countless colours, having reached the shores of the sea, took up its abode there. Then the illustrious son of Dasaratha, addressing Sugriva amongst all those foremost vanara, spoke unto him these words that were suited to the occasion, 'This army is large. The ocean also is difficult to cross. What way, therefore, commends itself to thee for crossing the ocean?'
At these words, many vain-glorious vanara answered, 'We are fully able to cross the sea. ' This answer, however, was not of much use, as all could not avail of its meaning. Then, some of the vanara proposed to cross the sea in boats, and some in rafts of various kinds. Rama, however, conciliating them all, said, 'This cannot be. 'The sea here is a full hundred Yojanas in width. All the vanara, ye heroes, will not be able to cross it. This proposal, therefore, that ye have made, is not consonant to reason. Besides we have not the number of boats necessary for carrying all our troops. How, again, can one like us raise such obstacles in the way of the merchants? Our army is very large. The foe wilt make a great havoc if a hole is detected. Therefore, to cross the sea in boats and rafts doth not recommend itself to me. I will, however, pray to the Ocean for the necessary means. Foregoing food, I will lie down on the shore. He will certainly show himself to me. If, however, he doth not show himself, I will chastise him then by means of my great weapons that are more blazing than fire itself and are incapable of being baffled!'
Having said these words, both Rama and Lakshmana touched water and duly laid themselves down on a bed of kusa grass on the seashore. The divine and illustrious Ocean, that lord of male and female rivers surrounded by aquatic animals, then appeared unto Rama in a vision. And addressing Rama in sweet accents, the genius of the Ocean, surrounded by countless mines of gems, said, 'O son of Kausalya, tell me what aid, O bull among men, I am to render thee! I also have sprung from the race of Ikshwaku and am, therefore, a relative of thine!' Rama replied unto him, saying, 'O lord of rivers, male and female, I desire thee to grant me a way for my troops, passing along which I may slay the Ten-headed (Ravana), that wretch of Pulastya's race! If thou dost not grant the way I beg of thee, I will then dry thee up by means of my celestial arrows inspired with mantras!'
And hearing these words of Rama, the genius of Varuna abode, joining his hands, answered in great affliction, 'I do not desire to put any obstacle in thy way. I am no foe of thine! Listen, O Rama, to these words, and having listened, do what is proper! If, at thy command, I get a way for the passage of thy army, others then, from strength of their bows, will command me to do the same! In thy army there is a Vanara of the name of Nala, who is a skilled engineer. And endued with great strength, Nala is the son of Tashtri, the divine artificer of the Universe. And whether it is wood, or grass or stone, that he will throw into my waters, I will support the same on my seabed, and thus wilt thou have a bridge over me, through which to reach Lanka!'
And having said these words, the genius of the Ocean disappeared. And Rama awaking, called Nala unto him and said, 'Build thou a bridge over the sea! Thou alone, I am sure, art able to do it!' And it was by this means that the descendant of Kakutstha's race caused a bridge to be built that was 10 Yojana in width and a 100 Yojana in length. And to this day that bridge is celebrated over all the world by the name of Nala's bridge.