The speaker is Atlantis, the legendary city, and it talks about itself in the first person. It begins with pride, calling itself the greatest of all cities, but admits right away that greatness doesn't last. Even the best will fall.
The middle stanzas show a city that is busy and useful, with messages coming and going and a purpose in solving people's problems. But that purpose is slipping. The speaker's powers are fading, earthquakes torment it, and its sanity is shaking, a clever blend of the city's literal trembling and a mind coming apart.
The last stanza looks ahead. Atlantis is famous now, but in a few years it will be remembered only as the city "drowned in his tears." The mood is sad and resigned, like someone who knows the ending and can't change it. The poem turns a myth into a small portrait of fear, exhaustion, and the worry of being remembered only for your failure.