A circle can represent continuity or wholeness. Symbols work on different levels. Just as they work in literature or art, they work in architecture. The Capitol's symbolism is not just in its design or physical details, but also in its openness. Fittingly for the emblem of American democracy, the grounds are unfenced and the building open to anyone who wishes to visit.
Free access is a symbol, too.
As a result, there is a constant underlying terrorist threat. Achieving the proper balance between security and access presents a difficult challenge. The threat is not just to the Capitol as a national symbol. Dignitaries from all over the world who visit the Capitol could potentially draw terrorist violence, and there is also the danger that an unstable, violent person may carry a political or personal grudge against an individual member of the House or Senate. Capitol Police officers are stationed on the grounds outside the building but there is no fence.