In our study of Ezekiel chapter four, we see how God uses the actions of the prophet to warn His people of the coming destruction of the city of Jerusalem. In chapter three, we saw God cause the prophet to be unable to speak with his mouth but he is going to act out the coming judgment as a warning. There were false prophets going around telling God's people that everything was going to be fine and that they were going to have peace and prosperity. In this setting, Ezekiel is called to tell the people the truth but he is told to do it in an unusual way.
"'Now, son of man, take a clay tablet, put it in front of you and draw the city of Jerusalem on it. Then lay siege to it: erect siege works against it, build a ramp up to it, set up camps against it and put battering rams around it.'" NIV translation
God gives the prophet specific instructions on what to do to demonstrate the coming destruction of the city. The clay tablet was basically a brick of dried clay that they used at the time because they did not have paper. In a siege, the enemy would surround a city and sorta starve the residents out. A ramp would be built up to the city gates and a big pole would be used to beat down the gates.
God was using this to let the people see that the city was going to be attacked even though the other "prophets" were predicting peace and prosperity. The same type of thing is going on today as many just tell the people what they want to hear and do not warn them that, like Jerusalem, judgment is coming to all the earth. This is what Peter was talking about, in 2 Peter 3, when he speaks of the scoffers in the last days.
"'Then take an iron pan, place it as an iron wall between you and the city and turn your face toward it. It will be under siege, and you shall besiege it. This will be a sign to the house of Israel.'" NIV translation
In our day of advanced materials, iron may not seem like a big deal but, in the Ezekiel's day, iron weapons were unbeatable. God has him to continue acting out His judgment by symbolizing a siege. This was a picture to tell the people that God had made up His mind and it was unchangeable. This is a lesson for us too as many think that we can go on doing whatever we want with no regard for the will of God. God has surrounded us with reminders of Him much like the siege works described. All of creation testifies to the fact that God exists and is in control.