The main topic or theme of this enigmatic poem would be the transient nature of a city's glory. Some historians argue that this poem (with its descriptions of bathing houses) is about the fall of the Roman city of Bath. Others argue that the poem is a commentary on the fall of the Roman Empire. Yet still others argue that the poem is about Stonehenge, with its descriptions of courts and walls of stone.
Whichever position you take, this poem is all about the loss of former wealth, glory, and power. Its main theme is the decay of a civilization in structural, political, and societal terms: sic transit gloria mundi 'thus passes the glory of the world,' or 'worldly things do not last.' You may notice that this poem praises the architectural structure of the city; stone built Roman cities are often lauded in Anglo-Saxon poems.
There in the olden time full many a thane,
Shining with gold, all gloriously adorned,
Haughty in heart, rejoiced when hot with wine;
Upon him gleamed his armor, and he gazed
On gold and silver and all precious gems;
On riches and on wealth and treasured jewels,
A radiant city in a kingdom wide.
Wondrously wrought and fair its wall of stone,
Shattered by Fate!
There stood the courts of stone. Hot within,
The stream flowed with its mighty surge. The wall
Surrounded all with its bright bosom; there
The baths stood, hot within its heart
what lines show the past splendors of the city?