This puzzle is very intriguing. I solved it on paper using X for squares that don't contain a letter, and the plus sign (+) to show that a square MUST have a letter. By using the process of elimination I discovered that Square 1-5 (Row 1, Column 5, lower-right corner) could not contain an X and therefore required A-D. Using the same method I found that Square 5-5 could not contain an X either, and after that it was fairly easy to correctly place all the letters. It may look difficult if you persevere you'll not only solve the puzzle but have the satisfaction of knowing that your powers of logic and deductive reasoning led you to the answer. Thanks Maarten!