I'm glad I watched this drama despite all the negative comments/reviews that I read through.
This is a lengthy review addressing each concern so it does have a lot of spoilers after #3 so don't read past #3 if you plan on watching without spoilers.

#1- This is a HISTORICAL drama. So keep that in mind before you say it's Anti-Japan or Japanese bashing.

You cannot deny the fact that Japan colonized Korea and had done terrible things in the past. Sure, everything is better now but doesn't change the fact that China and Korea were victims of Japan's brutality in the past. They tried to create a story line during that time period so yes, Japanese people will be painted as villains and it's inevitable.

#2- The drama was released in 2012? So keep that in mind when it comes to production.

If you're expecting to watch a high quality movie-like or dramas that you see currently, then don't watch it. It's a bit old and I think the quality is the same as any other drama that was released in 2012. I have seen about 2 other dramas around 2013/2014 and it's similar quality. That being said, some felt there were cheap scenes/production and I think it's because back then their budget was like that as well. The action scenes remind me of another k-drama I watched from 2013 and it's similar so I have no problem. I can't say the action scenes were terrible or cringe-worthy either. I've compared it to recent historical drama action scenes as well. There's slight differences but it's nothing major.

#3- The repetition of arrest, torture, interrogation scenes.

A lot of people had issues about the repetition of these scenes. Arrest & interrogation scenes are repeated because two of the main characters are Imperial Policemen so most of the time will be in the station. I think the interrogation scenes where the person eventually spoke while being interrogated were necessary. Interrogation was necessary for them to find answers. And even if some of them didn't give the answers to the investigator, they still spoke about their national pride and that was to probably show their bravery of enduring torture and not giving up or betraying their comrades. But there were some of them that didn't have the bravery nor could they endure the torture and spoke after a few minutes of torture. They could have probably limited the torture scenes where much wasn't going on. And for some who cannot handle torture scenes should probably skip it. The torture they actually gave back then was much worse than what was shown.

(Spoilers Below)
#4- They added too many characters into the drama and most of them weren't developed characters. Only the ML characters felt complete in a way to me. There were also decisions made by the characters that I questioned. A lot of the stuff could be cut down. The drama could have ended in 20 episodes, but they added too much to the story and dragged to 28 episodes.

I feel like they focused too much on Lee Kang To and Kimura Shunji's characters. There were so little scenes of Oh Mok Dan & Ueno Rie and it felt like they contributed very little to the story and drama itself. That's why I can't say much about JSY's & HCA's acting cause I think it's their poorly written characters. Not much about their background is known. And there's still a few mysteries about them. I wish they gave MD a stronger female role. I felt no attachment to either FL characters so it didn't even tear me up when she died. The only time I teared up were actually KT scene with his brother's death.

They did a really good job of developing Kang To's character who is in an identity crisis situation. He justifies why he must be a Jap's dog but when he loses his family and discovers his first love is alive, he starts to question his efforts and becomes a hero. From someone hated by both Japanese and Koreans to someone loved by the Koreans by being the Bridal Mask.
The same applies for Shunji. They developed him from a nice Japanese (which isn't so common) to someone slowly becoming monstrous. KT& Shunji's characters are so complex.
Things that I didn't like:
- How slow they made the characters appear. And how sometimes they don't think which just drags the story further.
Example 1: KT told MD there's a spy when MD learns of KT being Bridal Mask, they didn't think about dealing with Gye Soon as a spy?? A lot of the stuff could be avoided and not happen if they dealt with it later.
Example 2: It takes KT quite awhile to realize Shunji knows his identity. Till MD tells him. He doesn't assume or question why he was left alive after being wounded.
Example 3: Reporter Song knew the soldiers spotted him come out of Angel club. Shouldn't he have suspected the soldiers to interrogate why Angel Club let a man who is on a wanted list enter their club and not report to police? He should've sent someone to check the status and he would've known by Tasha that it's leaked and they should proceed with caution.
Also it's not first time Roh (waiter) blabbed his mouth to Police and try to cover up with a lame excuse. Why did Tasha still trust him with info?
Throughout entire drama Japanese police plants spies on Koreans but why don't they ever learn?