One key element is the conflict between the Assassins and the Templars. This drives the whole story. Another is the use of the Animus to explore past memories. Also, the Pieces of Eden are important as they are the source of power and the focus of the struggle.
The key elements include the historical settings which are very diverse. From the Middle Ages to the modern era, these settings provide the backdrop for the Assassins' and Templars' battles. The characters are also crucial. Each Assassin, like Altair, Ezio, and Connor, has their own story and motivation. And then there's the hidden knowledge aspect. The Assassins are often in search of ancient secrets that can help them in their fight against the Templars.