Behind Bars: Worlds Toughest Prisons

The most heavily-guarded prison in Croatia. The country's 500 most dangerous criminals serve their sentences here - and every day the struggle for survival begins anew. (S4 Ep2)

Taichung Men's Prison is the toughest in Taiwan, housing some of the country's most dangerous criminals and operating a system of "progressive correctional treatment". (S4 Ep3)

The high-security facility on the outskirts of Bucharest houses the most dangerous criminals, including murderers, rapists and human traffickers. Escape is near impossible. (Ep4)