Siege of Avalon is a traditional fantasy-RPG with an isometric view. The setting is an imaginary land which has been ruined by wars and it is up to you to defeat the evil warrior Mithras, who strives to conquer the entire land. Only the fortress of Avalon resists to his evil intentions.

The game is mainly story-driven, featuring many NPCs, and has action-based combat. It doesn't follow any particular set of RPG rules; characters level up after having gained enough experience from defeating foes. Class restrictions are rather mild, meaning that a fighter can learn spells and gain proficiency in magic, etc.

This game has been described as an "episodic computer game novel". It is divided into six chapters. The first one is available for free download, but you have to buy the others in order to complete the story. Second chapter takes the events from where the first chapter ended, while third, fourth and fifth chapters are sort of add-ons to the main story. The sixth chapter is dedicated to the final battle with the evil.

The game begins with an entry in the hero's journal. The siege has been going on for several years and his brother Corvus serves as part of the military in Avalon. When their father dies, the hero feels that it is his obligation to inform Corvus of the sad news, so he becomes part of the crew of a ship sailing from Elythria to Avalon to deliver supplies. The Sha'ahoul sinks all of the ships except the one the hero is on and the ship reaches Avalon without most of its supplies. It is not possible to make a return voyage to Elythria and so the hero decides to stay at Avalon and help out Avalon in whatever manner he can. The player gains control at this point and begins by speaking to Corvus and informing him about his father's death. The players then helps the people of Avalon with many quests and tasks, eventually becoming a knight, ranger, or wizard of legends.