Heaven is the dwelling place of God's throne, where Jesus Christ and His angels also reside. Jesus came to take those who believed in Him up to heaven at the time of the Rapture, where the marriage of the Lamb to His bride took place.
The city is a perfect cube, the height, width, and length are equal. This equals 12,000 furlongs cubed combined. A furlong is 582 feet, which makes 1323 miles (approximately).
The walls of heaven have 12 foundations and in them are the names of Christ's disciples. Each foundation is made of a different king of gem. Jasper, Sapphire, Chalcedony, Emerald, Sardonyx, Sardius, Chrysolite, Beryl, Topaz, Chrysoprasus, Jacinth, and Amethyst. The walls are also of jasper and the city itself is pure gold, "like glass", described by John in Revelation.