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.

2356. That 'Lot went out to them to the door (janua)' means that he acted cautiously is clear from the interior sense of 'the door' and of 'going out to the door'. 'A door' in the Word means that which introduces or leads the way either towards truth, or towards good, or towards the Lord. Consequently 'a door' in addition means truth itself, also good itself, as well as the Lord Himself, for truth leads to good, and good leads to the Lord. Such things were represented by the door and the veils of the Tent of Meeting, and also of the Temple, see 2145, 2152, 2576.

[2] That this is the meaning of 'a door' is evident from the Lord's words in John,

He who does not enter by the door into the sheepfold but climbs in by another way, that man is a thief and a robber. But he who enters by the door is the shepherd of the sheep. To him the doorkeeper opens. I am the door of the sheep; if anyone enters through Me he will be saved. John 10:1-3, 7, 9.

Here 'door' stands for truth and good, and so for the Lord who is truth itself and good itself. This shows what is meant by being let in through the door into heaven, and therefore what is meant by 'the keys' which unlock it.

[3] Here however 'a door' means a particular type of good that was suited to the disposition of those who besieged the house, for a distinction is made here between 'a door' (janua) and 'a door' (ostium). The former was on the outside of the house, as is evident from the fact that Lot went out and closed the door (ostium) behind him. This type of good was blessedness of life, as is clear from what follows shortly where he persuaded those who were immersed in falsity and evil. For such people do not allow themselves to be persuaded by actual good itself; indeed they reject it. From these considerations it is evident that here 'going out to the door' means that he acted cautiously.