To find the side length of the square, we know that the area of a square is given by the formula:
\[ \text{Area} = \text{side length}^2 \]
In this case, you are given that the area \(P^2 = 169\) square centimeters. To find the side length \(P\), you need to take the square root of the area:
\[ P = \sqrt{169} \]
Calculating that gives:
\[ P = 13 \]
Therefore, the side length of the square is \(13\) centimeters.