Threads of Time

Threads of Time (살례탑) is a Manhwa series in the era of Mongol invasions of Korea by Noh Mi-young. It is distributed in English by Tokyopop.

The plot premise is that Ordinary High School Student Moon-Bin Kim has difficulty sleeping due to a recurring nightmare where he's stranded over a thousand years in the past chasing a dark-haired girl into a deep abyss. Before long, the nightmare overwhelms him, and he is unable to tell whether he is Moon-Bin, a Seoul teenager at the end of the second Millennium, or Sa-Kyung Kim, the son of a prominent warrior family in the middle of the first Millennium. People in his present-day life assume roles in his historical life as he struggles to learn exactly who he is and what he's expected to do, straddling a transmigratory portal through time and space. In the present, his school's kendo club battles to stay in the championships but in the past, Moon-Bin finds himself at the threshold of a territorial dispute on the plains of Mongolia...

Tropes used in Threads of Time include:

Accidental Pervert - Moon-Bin didn't mean to walk in on Atan Hadas bathing... and he only had honorable intentions when he stripped her to treat her wounds. She didn't seem to mind.
Action Girl - Atan Hadas
Ax Crazy - Sali Tayi
Betty and Veronica - Chung-War (Betty) and Atan Hadas (Veronica). Only thing is, Chung-War is out of the game pretty early in the story after she gets kidnapped and forced to marry an enemy soldier.
Break the Cutie - Happened to Sali Tayi when he was young. His mother giving him away to enemy soldiers in exchange for the rest of the family's safety, and later not even remembering who he is.
Cold-Blooded Torture - Sali Tayi's torture of Moon-Bin's mother from the past. Slaughtering all the servants, cutting off her hand, and forcing her to watch as he rapes her daughter Sa-Lum.
Dark and Troubled Past - Sali Tayi's past.
Dude, She's Like, in a Coma - Sali Tayi's rape of Sa-Lum.
It's Personal - Moon-Bin wants to kill Sali Tayi partly because he wants to go home, but mostly because Sali Tayi killed his family in the past that he grew to love.
If I Can't Have You - Sali Tayi explicitly tells Atan Hadas that if she runs away with Moon-Bin and doesn't marry him, he'll kill them both.
I Want My Beloved to Be Happy - Atan Hadas eventually promises to marry Sali Tayi if he doesn't kill Moon-Bin. He doesn't take it very well.
Love Triangle - Sali Tayi loving Atan Hadas, who loves Moon-Bin, who likes her.
Longing Look
Murder the Hypotenuse - Sali Tayi comes to this conclusion very quickly when he discovers that Atan Hadas likes Moon-Bin.
Naked First Impression - Moon-Bin accidentally walks in on Atan Hadas bathing in a river. Surprisingly, despite her being a Tsundere, she didn't get angry at him.
Near-Rape Experience - Sali Tayi almost rapes Atan Hadas, but manages to stop himself.
Parental Abandonment - Sali Tayi's parents giving him away to enemy soldiers.
Reincarnation - near the end, it's revealed that Moon-Bin is Sali-Tayi's reincarnation. Also, the girl that Moon-Bin ends up with in the future is very obviously Atan Hadas' reincarnation.
Rescue Romance - The second time Atan Hadas meets Moon-Bin, he rescues her. Definitely the start of her romantic feelings for him. Not that she needed rescuing...
Revenge - Moon-Bin's great motivation against Sali Tayi.
Screw Destiny - Moon-Bin does this at the end, where he pretty much tells Sa-Lum that he is himself and not the reincarnation of Sali Tayi. It works.
Second Love - Moon-Bin's first love was his servant Chung-War. He later ends up with the reincarnation of his second love, Atan Hadas.
Set Right What Once Went Wrong - Apparently Moon-Bin was sent back in time for this purpose by Sa-Lum.
Single Woman Seeks Good Man - Atan Hadas towards Moon-Bin. He's seriously the most understanding nice guy you can find during that time period. This trope is furthered in that she hates bad boy Sali Tayi.
Stalker with a Crush - Sali Tayi is this way for Atan Hadas. As he puts it: "I have always been watching you..."
Time Travel - Pretty much the whole plot premise for the story.
Tsundere - Atan Hadas
Unlucky Everydude - Moon-Bin initially is seen as this.