i think a ward against magic would work where the villain in question was stupid enough to not replace the bars in a few years and the PC's can break down the bars easily (outside the cell magic is not warded as well) but the characters had their equipment taken so they have to take down the guards either with magic or fists, then they would probably take their equipment and proceed through to the confiscatory(where their equipment will PROBABLY be) with pretty low level equipment.