The pre-trial hearing is the first time the defendants have gone to the high-security court since February 2020.
Among them was the self-described architect of the 9/11 attack, Khalid Sheikh Mohammed. All five are accused of multiple war crimes.
The biggest legal block is the secrecy surrounding their detention before they got to Guantanamo.
"One of the most important issues in the case is how the torture of these men is going to ultimately affect the trial," said James Connell, defense attorney for Ammar Al Baluchi.
Critics say the men were subjected to interrogation tactics like waterboarding.
Much of the evidence remains off-limits to the defense.
for more features.