lordhoff: Now that I'm playing "1" and bearing in mind that I am relatively new to RPGs (just Stonekeep, Gothic I-III, Avadon 1-2, Vampire the Masquerade Redemption and Bloodlines, and Eschalon I-III), I've noticed many simularities between this game and Eschalon trilogy (not storywise) - is there some sort of relationship or is it just standard ways this venre is put together?
Well, I guess someone here would have to had played Eschalon to know what I'm talking about.
I haven't played Eschalon, but I do know that there is no relation between the Eschalon games and the Avernum games. In fact, I remember when the first Eschalon games came out, a lot of people were saying that it looked a lot like a Spiderweb game but with better graphics (and only one character instead of a party), even though it was from a completely different developer.
I would say that both Avernum and what I've seen of Eschalon are using some tried and true design tenets for the genre, though. I've heard that the original Exile games were heavily inspired mechanically by the Ultima series, and I wouldn't be surprised if the Eschalon games were as well. And much of that design, in turn, is heavily inspired by the Dungeons and Dragons pen-and-paper ruleset, at least for combat.
There are many types of RPG. The ones you've played show off a few of them: Stonekeep is a first-person dungeon crawler which would be very familiar to those who played classics like Dungeon Master and Eye of the Beholder. The Gothic games are open-world real-time RPGs of the type that are popular recently (e.g. Skyrim, Witcher 3). I haven't played the Avadon games but I hear they are inspired by Bioware games in terms of characters and combat design. Vampire: The Masquerade - Bloodlines is more in the vein of Deus Ex, i.e. a combination of RPG and first-person "immersive sim". And the Eschalon games are classic turn-based top-down (or isometric) RPGs like Ultima, or indeed Avernum.