First of all tell me a little about your series and introduce us to the sequel(s).
War of the Last Day series is an epic fantasy trilogy set in the world of Gorania, where mankind is on the brink of extinction. A cursed army possessed by demons descends upon the six kingdoms of Gorania, reducing entire cities and castles to ashes and bones. While the Goranians have no clue how to defeat such an invincible force, a glimpse of hope appears as a hapless trio takes upon themselves the impossible mission of saving the last of their kind.
Army of the Cursed, book 1 in the series, was an SPFBO Semi-finalist. The sequels Tattered Banners (released already) and Reign of Demons (to be released next June) take place in the same doomed world of Gorania where the war against the demons enters a starker phase as more Goranian kingdoms fall, leaving the fate of humanity in the hands of clashing factions.
The series has flawed relatable characters, a cool magic system based on a mix of alchemy and energy bindings, battle sequences that vary in types and scales. Also, scheming lords and corrupt soldiers, bandits and tavern keepers, forbidden books, and abandoned places full of secrets. And of course, a vast horde of immortal cursed barbarians 😊

You can get a copy from Amazon.

Do you find that most of your readers continue to read the whole series? Why do you think that is?  
I can’t say I have an accurate estimate, but I’m starting to notice that Tattered Banners is getting some positive attention recently. Based on the reviews that I received, I assume that Army of the Cursed readers loved what they read and they wanted more!

How difficult is it to add new characters in a sequel into already established relationships?
To be honest, this was never a big issue for me. If there is something that I take into consideration before I introduce a new character in a sequel, it will be its impact on the whole story. War of the Last Day has a huge cast of characters already—a crime I’m never ashamed of😁—so I always ask myself before introducing a new character: Is it necessary to do this? What happens to the story if I don’t bring in that character?

Is it difficult to continue with worldbuilding for a world you have already built in book 1?  Do you find it easier to switch locations for the sequel and start again with worldbuilding?
I guess worldbuilding becomes easier in sequels, especially when you lay enough ground in book 1. Actually, all the lore, facts, history…etc you use helps you think of new ideas and places to explore.

Have you ever been stymied by a worldbuilding or plot detail from book 1 that is very inconvenient to deal with or write your way around in subsequent books?
I don’t think I’ve faced this problem in particular 🙂. What I do remember is that several times I encounter a roadblock in the plot, and each time I figure a way out while I write. Was it because of some detail in book 1? I really don’t know if that has ever been the reason, but anyway, it is never a problem. Because I’m confident I can get my characters of that predicament as I did dozens of times before.

Would you say your craft has improved with the subsequent books?
I’d say that 100%. And if I’m to name one thing that has improved, it will be my plotting process. I guess my battle sequences got a little upgrade in my sequels as well, but I will leave that for my readers to decide😁

Do you have all the timeline planned for the full series?
Before I wrote a single word in the first book Army of the Cursed, I started working on the main milestones of the entire series. As the writing went on, I found myself able to zoom in on milestones that could be of less significance yet enough to advance the plot onward.
In other words, after plotting the whole thing, I plan/re-plan the next smaller parts of the story as I keep writing, That said, I rarely stick to everything I’ve planned. Many times, I have this last-minute idea that haunts my mind and tempts me to pursue it, which is something I’m starting to deal with as part of my writing process. You know, the plan is not obligatory, but hey, it MUST be there to feel reassured. 😄

Do you have any marketing tips for sequels?
One simple tip that some writers might forget: update the backmatter of your first book after you release the second. If your readers like your first book, there is a chance they will check the link to your sequel at the end of the book. If that link doesn’t exist, that chance—whatever it is—diminishes. 
But in my humble opinion, it’s all about marketing your great first book. Personally, I dedicate 95% of my marketing/promotional resources to book 1, bearing in mind that I might endure some loss for the sake of putting my story into the hands of as many readers as possible. If my first book appeals to a solid base of readers, I won’t worry that much about marketing my sequel. All I might do is say: hey folks, it’s out there! 😊