Philadelphia was located in the foothills of the Timolus Mountains, with fertile plains to the east. The city often suffered from severe earthquakes, which left it weak and poor.
This verse uses phrases from Isaiah 22:22.
key of David: Jesus the Messiah, God's chosen one, is the successor to David’s royal line. As the gatekeeper of heaven, Jesus has the authority to open and close the way to heaven (compare Matthew 16:19).