Fragility
Alicante, also called the City of Glass, can be considered as a symbol for fragility. Even if it is considered as being the safest place for Shadowhunters it proves to be the complete opposite. The walls that kept demons out for hundreds of years are taken down and even if it could have been prevented, those who run Idris refused to believe that such thing could happen. Alicante is the symbol for the old rules, a symbol used for the fixed ideas Shadowhunters have regarding the world. Because of their stiffness, both the old mentality and the city get destroyed, revealing that no matter how secure they seemed, they were extremely fragile.
Dreams
Dreams are common symbols used in literature regarding the communication between two different worlds. In the book, dreams get a prophetic meaning even if the characters that have them don’t realize that straight away. Clary realizes that some of her dreams that came to pass were actually visions sent by the angel kept captive by Valentine. Through dreams, Clary is sometimes able to see new runes, unknown by the Shadowhunter world.
Runes
Runes are a common element that appears in the book, just like in the previous two books. The runes that are used in City of Glass may be a reference to an ancient magic practice named kabbalah, which used the Hebrew alphabet as a way to summon greater powers. They believed that by using the Hebrew letters, they could recreate the language through which God first created the world and then attain the same power. In City is Glass, the runes are seen as something given by The Angel to the Shadowhunters, a type of language that granted them powers. Even if it is revealed that there are many more runes known only by Angels, the runes are still powerful.
Lake Lyn
Lake Lyn is used as a symbol for transition. Just like mirrors, lakes are commonly used in literature as being gates to mystical lands, otherwise inaccessible. Through Lake Lyn, Valentine is able to summon the angle who created the Shadowhunter race. Between Lake Lyn and the Mirror that appeared in the engravings portraying the Mortal Instruments appears to be two different objects in the beginning but it is discovered in the end that in fact they are the same thing.
The Mortal Cup
The Mortal Cup could be the equivalent of the Holy Chalice used by Jesus in the last supper. When Jesus used the Holy Chalice, he made a new covenant, thus creating a new group of people, loyal to him. Through the Mortal Cup, the Angel Raziel did the same thing when he mixed his blood with human blood and he created the Shadowhunter race.
