Perhaps you are wondering what a crenelation is. It is a parapet with short segments cut out. They are used in castles and fortress walls. So I could have also labeled this post "Battlements on LDS Temples" or "The Castellated Temple Style". But crenelation is a fun word, so I'm going to use it.
A major feature of the temples Brigham Young built are the crenelations and other castle style architecture used. The pioneers were trying to literally build the Kingdom of God and it showed in their architecture. Even churches were made to resemble castles. The defensive architecture was also used as a symbol of the protection faithfully living the gospel and attending the temple provided (temporally and spiritually). It also reinforced the royal imagery of a church with the royal priesthood and as sons and daughters of God destined to be kings and queens. It reminds us that the temple is the House of the Lord with Christ as the King of Kings, the Prince of Glory. The castle look also visually unites the four pioneer temples despite their differences.
pyramidal stones at the tops of the walls that make them look even more defensive. Another feature is the large stone retaining wall which includes crenelations and adds to the fortified look. The west end towers are octagonal giving a turret look, while the east towers are all square to make them visually appealing butting into a hill. This also makes the east towers appear weightier and stronger and larger, which is fitting as they represent the Melchizedek Priesthood.
this photo. The open ones are chimneys.
These four temples, the pioneer temples, are the only ones to use the castle style with crenelations. Other temples have mimicked features of these temples including the Brigham City Utah Temple which is currently under construction. That temple isn't using crenelations. The San Diego Temple is often compared to a castle and it is beautiful and has a look of strength, but it is not crenelated. The crenelated castle style is part of what makes the pioneer temples unique. I would like to see it incorporated into a new temple, but if not, we still have four superb examples of the style. I should also mention that using a crenelated castle style of church architecture is not unique to Mormonism - I saw many examples of it on my mission in England.