So while Joab had the city under siege, he put Uriah at a place where he knew the strongest defenders were. (NIV)
So when Joab was besieging the city, he assigned Uriah opposite where he knew the enemy's most valiant men were. (AMP)
And it came to pass, when Joab observed the city, that he assigned Uriah unto a place where he knew that valiant men were. (KJV)