What does Daniel 6:17 mean?
The den mentioned here was probably a modified cave or a brick structure designed to hold captured lions. Based on this passage, there was probably an opening at the top to allow condemned people to be literally "thrown" inside. Either this top door, or a ground-level entrance where armed men could come in for cleaning, is what the king seals. Using the royal seal left no doubt that Darius (Daniel 5:31) was making good on his own decree (Daniel 6:6–9), though it turned out to be a trap for his trusted advisor, Daniel (Daniel 6:10–15).It's possible the conspirators who hated Daniel insisted on this security so that Darius could not rescue Daniel. The conspirators knew Darius planned to promote Daniel to a position above theirs (Daniel 6:1–3). Whatever the case, the conspirators failed to account for the power of Daniel's God.
This scene vaguely foreshadows what would happen centuries later at Jesus' tomb. When His body was placed in the grave, a huge stone blocked the entrance (Matthew 27:60).