Age of Decadence is one of these games that, despite generally favorable reputation, is grossly underappreciated.
It's a great game in many ways, and it's one of the most unique RPGs you will ever play.

To say it is "good" is to say nothing. There were multiple games that were released after AoD that tried to do what AoD did and yet came nowhere close to it. Tyranny specifically comes to mind - a good RPG regardless but even when it's praised, it's usually for the "you play as the bad guy" aspect as opposed to its truly praiseworthy take on non-linearity. Well, it's because that aspect is honestly speaking pretty shallow.

If you ever played an RPG and thought to yourself: "Wouldn't it be great to see how those huge worldbuilding events could play out were I to be involved in them", Age of Decadence is the game for you. I haven't ever seen a game, let alone an RPG, achieve this degree of true non-linearity. The whole point of the game is that you will affect the story. No, it won't be just a slideshow of "that had happened" at the end of the game. No, it won't be just a mostly pointless choice during the final moments of the game that allows you to experience a couple of different endings. No, if anything, Age of Decadence is at very least three games bundled in a single package.
You could play the game for its (unique and well-executed) combat and get involved in every major battle happening throughout the story, possibly changing the outcomes of those battles.
You might come to appreciate the characters, dialogues, and actual text quests more and avoid getting involved in the combat at all, resulting in the game playing more like a visual novel of sorts instead. Mind you, the aforementioned battles? Oh, be sure they will still happen even without you. You will hear about them and might even have to suffer their consequences but they are a part of this world and you have made your deliberate choice to avoid them.
Or maybe you will find that the whole world makes no sense and there is something much, much deeper going on and maybe, just maybe, you will want to try digging through all the secrets and lore of this world to finally get to understand it and see the actual truth.
The choice is yours. And this time around, there are actual important choices to make.

Of course, the game isn't perfect. Nothing is. But I think diving in while being aware of the game's quirks and arguably shortcomings results in a great overall experience.
The game is short. Especially by RPG standards. That is to say, "a single playthrough doesn't take long". It's a very important clarification because Age of Decadence is actually intended for several playthroughs. And, as it was said before, each one of these "playthroughs" can be a completely unique experience to the point of feeling like a completely different game.
As a natural extension of this issue, the game will refuse to "allow you to experience everything". Implying "in a single playthrough". The game does expect you to realize during its introductory stages that you can't really have it all and you will have to give up on some things. It's just mathematically impossible in this game to be the best combatant there is, the most shrewd "know-it-all" street rogue to ever live, and the most gifted and pleasant silver-tongued merchant who could make living by selling sand in the middle of the desert - all at the same time.
Yes, it makes sense if you think about it but that's not really the way most RPGs handle this kind of thing. Yes, you can occasionally miss out on a (mostly flavor) option here and there but you are highly unlikely to get locked out of a huge and important part of the game because of your choices. Age of Decadence will lock you out of the content. That is just something you have to accept and preferably be ready to experience.
As soon as you drop the "I must experience everything NOW" mindset and stop restarting the game every twenty minutes because there is yet another skill check you've missed because you've distributed your skills "wrong", the game will click for you.

Take it for what it is - a short but elaborate story of a single important life that somehow changed the history of this intriguing, complicated, and deep world.
How exactly? Well, that's for you to decide™.