The Harp
In "The Harp of India," the harp is symbolic of India's past and how it lost some of its pride and identity while operating under British rule. The British suppressed the Indian people by silencing and neglecting them, preventing them from becoming themselves.
War
In "The Maniac Window," war is symbolic of the way that many countries get their subjects (in this case, the narrator of the poem) to fight in war as a way to display patriotism and get amped up. But the narrator of the poem, who had fought in wars for his country, no longer has a "warrior heart," showing that he no longer buys into his country and their reasons for fighting.
The Tomb
The tomb mentioned in the title of "The Tomb" is symbolic of the way that many immigrants and explorers hide themselves and what they truly want in life as a way to appease their totalitarian rulers. After they arrive in the new land, where they hope freedom of expression will be more plentiful, they often find themselves quite literally in a tomb: dead from bad conditions.
The ruins
The titular ruins in "The Ruins of Rajmahal," are symbolic of the way that empires across the world (but especially the former British Empire) exerted control over their subjects. The poem talks about the "ruin reluctantly leaving," reflecting the situation those countries dealt with.