איה סופר המגדלים שהיו שונים שלוש מאות הלכות במגדל הפורח באויר
"Where is the one who counts the towers - for they would teach three hundred laws concerning a tower that floats in the air"
The artscroll note says that this is referring to the shape of the Lamed, which looks like a tower that is falling. Thus, many write the upper part of this letter slanted, as if it is falling. See here in full: