Complex sentence: The concert had what one contributing source calls a "Field Of Dreams" effect when without promotion, the Millennium Stage hall is said to have filled beyond capacity (in excess of 1250 in attendance were tallied).
Split into simpler sentences: The concert had what one contributing source calls a "Field Of Dreams" effect. Without promotion, the Millennium Stage hall is said to have filled well beyond capacity (in excess of 1250 in attendance were tallied).