Step 1:
This is a 20x20 cm paper.
Prefold and cut in half.

Step 6:
Fold over these two prefolded lines.

Step 5:
Fold both parts in half two more times. Unfold the model afterwards.

Step 2:
The building is made out of two different models, both folded out of a 10x20 cm rectangle.
Prefold in half, then fold both edges to this prefolded centerline.

Step 4:
Prefold in half, then fold both edges to this prefolded centerline.

Step 3:
Fold both edges to the centerline and unfold the model afterwards.

Step 10:
Fold these two edges into a straight angle, repeat on the backside.
Finally, glue the edges together, if necessary.

Step 8:
Fold the model flat again, the mountain-fold is a prefolded line.

Step 9:
Fold the left and right part straight downwards, use the prefolded lines as guidance.

Step 7:
Fold these two spots inwards, see arrows.

Step 15:
So, each column is made out of two large pieces and one small piece on top, see the next photo.
The columns are placed into a round or oval shape, see photo below.

Step 12:
The upper model of each column is a little smaller.
Repeat steps 2 to 8 on another 10x20 cm paper.
Then fold over these two lines, they're in the middle of two prefolded lines.

Step 11:
Done....

Step 14:
Fold the edges into a straight angle, the folding lines are in the middle of the outer edge and a prefolded line. Repeat both folds on the backside.
And glue the edges together.

Step 13:
Fold the left and right part straight downwards.

Step 21:
Fold the four corners inwards and the edges straight upwards. Use glue or tape on the corners.

Step 19:
I made three of these tribune parts

Step 20:
The other 10x20 cm paper is for the stage. Prefold over these four lines.

Step 18:
Fold the left part into a straight angle.

Step 17:
Repeat step 1 on another 20x20 cm paper.
Then repeat steps 4 and 5 on this 10x20 cm paper.
And fold these prefolded lines into a straight angle.

Step 22:
Instructions for the two greek columns with roof are in the video.