Humans are created in the image of God and reflect aspects of God. Here that aspect is ruling.
God rules
Ruling reflects the role of a king.
But ruling is also delegated to people. We are given authority to rule. God rules through people - submitted, obedient people.
We have been given stewardship/management of God's world.
The Kingdom of God exists when we exercise proper stewardship/management/oversight/rule
See this list of parables. To find those that refer specifically to the Kingdom of God, select the "Kingdom of God" in the Themes filter.
The disciples ask about the parables (plural) and Jesus says that the secret of the Kingdom is revealed in the parables (plural). Although some of the parables refer specifically to the Kingdom (e.g. "The Kingdom of God is like...") this suggests that the parables generally were teaching about the Kingdom of God.
Jesus charged the disciples with proclaiming the Kingdom of God (v.2) (along with healing the sick and delivering the demon possessed). The disciples are then said to proclaimed the good news and healed people everywhere (v.6). We can either assume that they were disobedient or that "the good news" is synonymous with "the Kingdom of God".
The wealthy man asked Jesus what he had to do to get eternal life. Jesus responded talking about entering the Kingdom of God (v.24), suggesting that the two are synonymous.