The construct

The construct was a prison outside time and space.

Features
The construct was made out of the very fabric of time itself. It was located outside the normal universe, making escape nearly impossible. It contained six featureless chambers of differing sizes. Time didn't pass inside the construct, though death was possible. If the construct was breached, the inhabitants would be exposed to the time winds. (PROSE: Freedom)

History
The construct was made by the people of Arbrocknel to house their worse criminals. It was eventually abandoned, though the Master somehow learned of it.

set up the Freedom Corporation to create the Freedom 1b time machine. When he was captured, the Master had Gerald Gooder (his artificially grown servant) hold a press conference for it. When the Third Doctor and Jo Grant began to investigate, the Master used Gerald to lure them to the Freedom 1b, which then transported them to the construct, leaving them both trapped.

Though the Doctor and Jo were trapped for an indefinitely long period of time, they were eventually freed when the Master's next stage of the plan involved destroying Earth, forcing the Time Lords to intervene. They sent the Doctor's TARDIS into the construct and restored the Doctor's memory on how to pilot it. (PROSE: Freedom)