The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

In the ancient world cities were nearly nations unto themselves – they existed within walls, with their own laws and customs, generally centered on the common purpose of trade. This is not as much the case in the modern world, but we still tend to divide ourselves city by city in terms of sports teams, accents, music and culture, and still tend to generalize the character of people from other cities. It follows, then, that in the Bible cities represent various doctrines – collections of inter-related ideas about spiritual reality. Such doctrines can be based on anything from the Lord’s true teachings – the New Jerusalem seen by John in Revelation – to the falsity and heresy of Sodom and various cities obliterated by the people of Israel at God’s command. On the most personal level, a city can also represent the natural mind of one person – which makes sense since we each to some extent have our own doctrine and our own set of ideas and beliefs.