First of all: This is long. It is dragged out quite a lot – I watched this on Netflix, and ended up watching a lot of the episodes on 1,25 speed. Couldn't do faster because of the subtitles, lol.

Second: It is a bit dated, but sometimes that doesn't matter that much with historical-style dramas. I try to overlook the dated factors in my ratings for older shows, but here there were elements that slightly bugged me to the extent that I will mention them:
- (Occasionally) Poorly done makeup, especially the wig-lines
- Overly dramatic music, a bit repetitive
- "Soap-operaish /telenovela" style

When it comes to the casting and acting, I think they did a decent job under the circumstances. It tended to be "overacting" for certain characters (especially the villainous ones), which I am no fan of.
The three main leads all did rather well – I found the performances to be a bit either too flat or too stereotypical at times, but unsure if that has to do with when this was produced or the script.

As for the story, it became extremely plot-heavy and too dragged out, especially the last like 10 episodes or so. I nearly abandoned this after watching somewhat over 30 episodes, but then it really would have felt like a waste of time.
There were some smaller things in the story that either didn't quite add up, or that I missed.

When about half-way in this drama, it reminded me a little bit of "Empress Ki", but that is a much better drama.
As for this drama ("Introduction of the Princess / The Glamorous Imperial Concubine"), I get the impression that they tried a bit too hard to make it into an epic, historical drama.