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I can't remember the name, but key points include:

-the tower was a way of making heroes to repel against an invasion of earth by some sort of cthulhu/evangelion monsters. no one involved knew this.

-after they exited the tower, they were unable to grow any stronger.

-the tower had 3 levels: easy, medium, and difficult. The difficulty could be chosen upon entry to the tower, and the more difficult the mode the stronger the survivors were upon exit. they did not know this at the beginning.

-whilst in the tower, they could talk to each other using some sort of forum. the mc did this on his second regression so he could share tips and hopefully increase the number of survivors.

-the mc chose easy the first time, but difficult the second time. the difficult mode gives each person their own tower without other humans in it. other modes involve a shared map of sorts.

-at certain defined points in the tower, the humans exit the tower back to earth and must repel monster invasions before being transported again to the tower.

Any help finding this would be appreciated. I have too many bookmarks and too few notes. Thank you.

It's a villainess manhwa set at the ending of the story. The villainess avoided the death flags by befriending the heroine and marrying the 2nd male lead. It's the villainess' wedding day, and she has to get married at a specific castle on a specific day for some reason or she'll die. Her fiance runs off with the heroine, and the villainess has to find someone new to marry immediately. She marries someone else, and the fiance is salty about it for ages, which is the point. Something something revenge?
I don't think I read too far into it. If anyone knows which manhwa this is, it would be appreciated if you would let me know. Thanks.