Terme Achilliane consist of several structures partially covered by the cathedral. Reached through a trap-door in front of the church, it is dominated by a wide rectangular hall with four pillars in the centre which support the vaults. The ceiling was decorated in stucco with representations of putti and vines. 

Many other rooms surround the pillared  hall, suggesting that the complex was of considerable size. It was perhaps built by the governor of Sicily, Lusius Laberius at the beginning of the third century AD, as indicated by an inscription (now lost) which stood out on the façade of the cathedral prior to the earthquake of 1693.