The Ultima series were instrumental in the development of the Western RPG as we know it today. Despite their age, they still have few rivals in terms of depth, challenge and ambition -- and this became particularly apparent in the "Age of Enlightenment" trilogy which comprised the fourth, fifth and sixth games in the series.

While Ultima 1, 2 and 3 were fairly simplistic combat-heavy affairs, Ultima 4 introduced the Avatar, a character concept under which the player would enjoy the remainder of the series. Rather than being a typical "save the world" hero, the Avatar was supposed to be the embodiment of the Eight Virtues, providing an early take on the morality systems we take for granted in our Western RPGs today. This also enabled the games to take on deeper stories than simple "Bad Dude Needs Killing."

Ultima 4 is already available for free on Good Old Games, but has been included in this new trilogy for continuity's sake. Therefore, for $5.99, you're effectively paying for Ultima 5 and 6, where the series really began to hit its stride. Ultima 5 introduced more in depth NPC interaction, day-night cycles and more varied map layouts. Ultima 6 saw developer Origin Systems' storytelling abilities hit a new high with an ambitious plot exploring human prejudice and ignorance.

That leaves three more titles in the Ultima series to complete it -- Ultima 7, widely regarded by many as the best in the series; Ultima 8, which dispensed with the traditional top-down view and party-based mechanic in favor of solo isometric exploration; and Ultima 9, the "black sheep" that effectively killed the series but, despite its flaws, set the template for modern Western RPG titles such as The Witcher.

Grab the Age of Enlightenment trilogy here for $5.99 and just 14MB of your valuable hard drive space.

This article originally appeared on GamePro.com as Good Old Games Continues Ultima Revival with Ultima 4, 5 and 6 for $5.99