The only time you get new skills (ones you didn't have any score in previously) is when you choose classes at 10 and 20. Your rating in the skills you have goes up, but no new skills spontaneously become available.
In the 10-19 range the only class that can use tower shield is Warrior. I don't know if the crusader derivatives (Paladin and Shadowknight) learn tower shield at 20, but they're limited to kite and under as crusaders. The highest any other class gets is round shield, I believe (not 100% certain on this).
Kurak - lvl 13 crusader, Antonia Bayle