Both of the guards have a handle. If the guard that always lies pulls his handle, then the minecart will divert to the second track. If both guards pull their handle, the prisoners will be released. One of the handles is covered by a wooden box, where half of the wooden planks of the original box have been replaced by new ones cut from a tree that fell when nobody was around. If guard A is taller than guard B, are we getting paid for this quest?
But does the DM have a contingency plan if someone casts Fireball?
Guardian portraits, because whoever wrote the "ordinary-looking furniture with counterspell" statblock was presumably really fed up of their sorcerer player