Most courthouses have a distinct architecture which embodies the ideals of truth and justice.
In To Kill A Mockingbird, Harper Lee describes the columns in such a way they transcend time. Columns are a fundamental structure in ancient Greek and Roman architecture. Over time this architecture came to represent what American freedom "looked" like. It is no accident the Capitol and Supreme Court look the way they do.
The Lincoln Memorial's columns are a distinguishing part of the structure. The thirty-six columns represent the number of states in the union at the time of Lincoln's death. In 1939, Marian Anderson an African American woman sang "My Country, 'Tis of Thee" on the steps of the Lincoln Memorial because no other Washington venue would allow her to. In 1963 Dr. Martin Luther King Jr. had a dream, it was delievered on the steps of the Lincoln Memorial.The columns are filled with "humanity". Lincoln was referred to as "The Great Emancipator", the same could be said of Atticus Finch's defense of Tom Robinson.
The columns of the courthouse are symbolic because they do not represent truth and justice. To represent truth and justice, the columns require humanity. Atticus Finch's defense of Tom Robinson represents that humanity. Prior to the Civil War the columns were just columns. Without humanity for our fellow man, columns just hold up the roof.
Teh columns on the Maycomb County Courthouse were remnants of a previous courthouse. Scout says they reminded her of Arlington (Robert E. Lee's home), because they were bigger than fit the construction of the new courthouse.
The columns therefore symbolize the New South, which was hanging on to remnants of the Old South (the columns), even though those attitudes, belifes, and prejudices did not "fit" with the Southern society that was developing (slowly) after the Civil War. As an attempt was made to make the new courthouse fit the old columns, many Southerners were trying to make the New South still supported by antebellum values.
From the description of the courthouse in chapter 16, we know that the columns on the courthouse were all that remained of the original courthouse that had been burned down in 1856. Those columns dated back to pre-civil war days and they symbolize the glory days of the south: the days when blacks had been slaves. This was in juxtaposition to the Victorian courthouse that had been built around the older columns. They attempt to preserve the columns of the old courthouse also symbolize the desire of Maycomb inhabitiants to hold on to the past, even when the past was useless or not needed any more, just as the columns were not needed to hold up the court house. I also find it interesting that the big Greek revival columns also mirror the type of column found on the White House, The Capitol and the Supreme Court buildings. Perhaps they also symbolize America and justice which will be denied in that courtroom which is reflected in the fact that the original courhouse burned down in 1856, when there was no justice for Southern slaves.