Possible Clues:

• See 19
• It has four equal sides and four right angles
• Old-fashioned place in France
• Equilateral quadrilateral
• Out-of-date on all sides
• Assault on crossword grid carried out with military precision?
• Old school quadrangle
• Four-sided regular polygon
• See 2
• With 43-Across, what the circled letters in the grid form