Secret of the Tower — Debra Doyle; James D. MacDonald

What's a wizard without magic?
Randal broke his promise -- the vow that all apprentice wizards must take never to use a weapon. Now Randal can graduate from the School of Wizardry only on one condition: that he not use magic until he is pardoned by a master wizard.

Randal must travel to the wizard's faraway tower ... a journey made all the more perilous because he may use neither sword nor magic for protection.

When Randal finally reaches the mysterious tower, it appears to be abandoned. But he soon discovers that the structure holds a deadly secret....