Daniel 6:14 & 15

"When the king heard this, he was greatly distressed; he was determined to rescue Daniel and made every effort until sundown to save him. Then the men went as a group to the king and said to him, 'Remember, O king, that according to the law of the Medes and the Persians no decree or edict that the king issues can be changed.'" NIV translation

The king did all that he could to save Daniel but even he could not change the law. The king was at risk if he did not carry out the punishment that was prescribed by his own decree.

This is a picture of the hopelessness of man under the Law of Moses. Like the law of the Medes and the Persians, God's Law could not be changed even by God.  

Daniel 6:16 & 17

"So the king gave the order, and they brought Daniel and threw him into the lions' den. The king said to Daniel, 'May your God, whom you serve continually, rescue you!' A stone was brought and placed over the mouth of the den, and the king sealed it with his own signet ring and with the rings of his nobles, so that Daniel's situation might not be changed." NIV translation

The king obeyed his own decree and had Daniel thrown into the cave with the lions and even "prayed for him". The entrance to the cave was sealed by a large stone and the seals of the king and the nobles. They did this so that nobody could let him out without their knowing about it. This is a picture of what would happen when Jesus was taken down from the cross. His tomb was also sealed with a stone and guards were even placed outside.