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Seventeen-year-old ALLII is Princess Albalia of Sallonia , a land with two distinctive features – extreme climatic conditions and vast deposits of Sallikan, the world’s most precious metal. Ignored by her father, Allii is befriended by her beloved stepmother, BELLIZZA. Bellizza helps Allii avoid a distasteful marriage. Allii stands by Bellizza when the king and the court turn against her due to her inability to give birth to the much wanted heir to the throne. Then Bellizza is poisoned and all evidence points to Allii. She is incarcerated and denied access to her father and to NANA, the beloved nurse who mothered her. She escapes and flees Sallonia, with the help of a daemon-dog. She is determined to avenge Bellizza and prove her own innocence to her father by finding the real killer. Since she has no access to the murder scene or witnesses, she uses unorthodox means to gain the information she needs. 
From a woman who lives in a bubble outside time, Allii learns the history that is not in the approved history books. Sallonia once belonged to a race called Muffics who lived close to nature. It was invaded by warlike Sallonians who believed it was their god’s gift to them. Muffics were expelled to the margins of the land; friendly interactions between the two groups outlawed. Allii’s father, a Sallonian prince, and her mother, a Muffic woman loved each other. They eloped and lived together though laws prevented their marriage. Therefore Allii is a bastard and a Halfling, an ‘impure-bred’ despised by both sides. When circumstances forced her father to become king, he abandoned her mother who died soon afterwards. 
From a cousin who made a forbidden union with a Muffic woman Allii learns of the succession-crisis in Sallonia. Chief Minister SHERRIZ and Chief Priest PENA want the king to make a new marriage. The king’s cousin BERNALIN wants to change the laws so that Allii can succeed her father. Another cousin, ROCKRIN and his eldest son BROVNALIN angle for the throne. Halflings plot rebellion. 
From the secret letters Bellizza wrote to her sister Allii discovers that her stepmother had been resisting attempts to divorce her when she was killed.
While Allii tries to make sense of the information she has gathered she hears that her father has been killed and Nana locked up. Rockrin is now king. He plans a war of annihilation against Muffics to unify all Sallonians behind him; and to grab the Muffic lands which are believed to contain the last untapped Sallikan ores.
By putting the clues together Allii concludes that Bellizza was killed by Nana. She manages to reach Nana’s prison and asks her why she killed Bellizza. Nana replies that she wanted the king to remarry and father a non-Halfling heir. She had no idea that her co-conspirators (Sherriz, Pena and Brovnalin) planned to scapegoat Allii. Horrified, she revealed the truth to the king who decided to expose the killers and exonerate Allii. Sherriz and Pena then aligned with Rockrin and murdered the king. 
Rockrin surprises Allii and Nana. He confirms Nana’s story. When he tries to kill Allii he is killed by NELLIN, Bellizza’s chief lady-in-waiting. 
Pena and Brovnalin arrest Bernalin and stage a public show-trial. Allii reveals herself and tells the court and the crowd the truth about who murdered her father and stepmother. She is backed by Nana and Nellin. The crowd turns against Pena and Brovnalin. Sallonian army, consisting mostly of forcible-conscripts, falls into disarray. With Halfling rebels on the march, Sallonian elites agree to make Allii queen.
The quest to uncover Bellizza’s murder has also been a quest of self-discovery for Allii. Sallonia is home but she doesn’t feel at home there. Her father betrayed her mother and stepmother because he prioritized the crown. She won’t go the same route. When Nellin kills Nana, in revenge for Bellizza’s death, and submits herself to Allii’s justice, Allii walks away sick at heart.
Allii refuses the crown. At her insistence, the conclave comes to a tentative agreement to replace the kingdom with a republic.
Having done what she could to set Sallonia on a less unjust path, Allii leaves. She has realized that she has a talent for at uncovering what is hidden. She will roam the world, investigating mysteries and helping those who are unjustly persecuted. Her birth will not determine her fate; she will. 
This is a little confusing. I would just say “Seventeen-year-old ALLII is the princess of Sallonia…”
These details are interesting, but don’t seem to be integral to the plot. If you’re trying to cut this down to one page, you can omit these details.
Isn’t Allii the heir to the throne?
What are these unorthodox means? Be specific!
Wouldn’t Allii already know some of this? She knows Bellizza isn’t her real mother, so she must have questioned who her mother was. Was she lied to? Was she led to understand her real mother was Sallonian?
These are all interesting details that add a lot to the main plot, but you need to omit all or most of these names. It muddles the reading. You can rephrase this as “ The Chief Minister and Chief Priest want the king to make a new marriage. The king’s cousin wants to change the laws so that Allii can succeed her father. Another cousin and his eldest son angle for the throne. Halflings plot rebellion.” That way, it’s a lot clearer who’s doing what without getting embroiled in name soup.
I would add “the king’s” here to clarify who’s divorcing who.
Again, I would just say “Allii’s cousin” here.
I got a little confused by the end of this paragraph. Go back through and reword, making sure to focus on who’s doing what, and what the consequences are for the main plot.
You might want to keep Rockrin’s name, since he recurs in the synopsis, but you definitely don’t need to name Nellin.
This is a nice ending and good wrap-up for the plot.
You’ve got some Name Soup going on here. Limit the proper names to 4-5, tops, to make sure the reader doesn’t get confused. Here, I’d use Allii, Bellizza, Nana, and maybe Rockrin. Everyone else can be described by their relationship to Allii.
It sounds like a good plot with a lot of action. With that in mind, trying to condense it into a short synopsis was bound to be tricky. I’d go back through and make sure each major plot event is descibed fully (but briefly), in separate sentences or even paragraphs when necessary. Remember, a synopsis is a map of the plot, so you need to make sure the reader can follow the plot from Point A to B to Z.
Overall, nice job, and best of luck with this book!