The Rebels' Assault — David Grimstone

Decimus Rex has escaped Arena Primus in the company of a fellow slave, Olu Umbika. Together, they manage to board a slave ship and sail away from the trials and Slavious Doom. But now their four friends are being used as bait to get them to return. Decimus and Olu must reach the Suvius Tower to rescue their friends before they get captured themselves!