If you're familiar with Spiderweb Software's RPGs, it will come as no surprise that Avadon: The Black Fortress is another extensive old school party-based trek through areas full of nooks and crannies with unique encounters and secret loot caches to be had, as well as exciting set piece encounters and an intriguing overall narrative. That's pretty much true of everything they put out. The big changes to the formula are: a new world; a significantly more linear progression than most of the other entries in their lineup, a class-and-skill-tree approach to character builds, and party members who are fully fledged NPCs that contribute insights and dialogue throughout the plot as you move through it, can be spoken to between missions, and have their own side agendas you will have to deal with.
The new world is an intriguing fantasy setting where five disparate and feuding lands have banded together in a defensive pact against pirates, invading empires, marauding humanoids and dragons, a blighted magical corruption, and other exterior threats. The pact is held together by the dictatorial enforcement of the Black Fortress Avadon, and they maintain a network of spies and enforcement agents to ensure compliance. The fairly unique conceit of this particular entry (and who knows, perhaps followup games in the series) is that you play Hands of Avadon, sent to enforce the laws of the pact and defend it from threats within and without. This gives you rather more authority and a much more fearsome reputation than your typical band of wandering adventurers, and this is used to good effect throughout, although alas you still have to contend with buying your own supplies, a tired RPG trope that seems even more out of place in this context (though they do attempt to explain it).
The linearity is somewhat unfortunate, but things do open up as you gain access to more of the areas and return to zones on new missions, and most of the sidequests can be done at your leisure. Still, the flipside of linearity is that you're always being sent into challenges that you should be equipped to handle, and it's always clear what you're expected to do next, neither of which are guaranteed in more open formats.
I am personally a big fan of Avadon's approach to skills and classes. I find the structure of new ability at rank 1, additional effect(s) at later ranks, and unlocking a secondary ability at a late rank very rewarding and I never feel like my points are wasted or that I have to save points from level to level just to advance at all, which have been issues I've historically had with the more freeform skill approach earlier Spiderweb games took (and other games with skill systems, for that matter). And Avernum in particular suffered from boring fighter syndrome where spellcasters got huge suites of spells to cast for a varied combat experience and fighters...got to hit things. Not here. The Blademaster class even passively is a blast, with heavy passive investment rewarding cleaves, a hilariously effective level of hit prevention, and the chance to riposte, curse enemies, or begin regenerating if hit, and then on top of that gets group buffs and debuffs and attacks with additional secondary effects. The Shadowwalker hurls shurikens, teleports around, throws pots of acid and smoke bombs, backstabs and poisons, and is a huge fountain of damage in general. (Needless to say, I was pleased when the Avernum remake cribbed from this system, albeit in a more Avernum-y way.)
As for the NPCs - there's one of every type, you get them all pretty nearly immediately, they stay levelled with you even if you're not using them, and they all have interesting personalities and backstories. I've never been a huge fan of the "party too large for everyone to simultaneously adventure with you" thing, but this is about as good as it gets considering that.
So all in all, another classic RPG. It's not quite as old school as previous series from Spiderweb, but the deviations from that formula mostly are in the vein of improvements in RPG design, in my book, not stepping away from the core appeal.