I plan on reading dungeon defense but I don't know whether it is a completed series can anyone tell me and give me your opinion about it
No, it's just up to date with the raws. Translations will continue when author release new chapters, and i think there might be an ETA on the translators site. But you can read it right now, you'll just have to wait for new raws unless you're lucky and finishes at the same time as the author
It's not completed and likely won't be for a long while, but imo it's fantastic. It's a lot darker than most LN, so keep that in mind. There's a fair amount of (usually not super descriptive though) raping and killing.
It's a light novel, not a web novel, so it's released in printed volumes, so you have to wait between releases, between less-than, and around a year. But the translator does the translations really quickly considering how good quality it is, and how long the chapters are. Right now, the 5th volume was translated long ago, so I'd expect vol 6 in a few months. But we're quite a bit away from the completion. There is still a while before the prologue of vol 1, which was 2 years into the story.