The Tower of Doom is a dungeon that contains 50 floors filled with enemies. There is a challenge for each floor you do. You must defeat the incoming enemies before the timer reaches 0.
Once finished and completed the challenge of the first floor, you will enter a room that will lead you to a staircase, ascending you up towards the higher floor. As you ascend to a higher floor, the monsters become harder to defeat and the challenge becomes harder to beat. But also the rewards are better!
Tower of Doom Guide – Lost Ark

The Tower of Doom is a long-lasting event. By a long-lasting event, I mean that it will take a lot of time if you wish to reach the last floor and collect those wonderful rewards. Always be prepared fully once entering this dungeon.
Bring enough potions that will last you even longer than the dungeon is. It’s always good to have extra. Set up your proper cards for the playstyle that you wish to play during those 50 floors. Once you are in there, you are alone. Also go to repair vendors and go inside the dungeon with repaired gear, even an extra set if things don’t go as planned.
Having the appropriate set of skills will boost your overall progress in the dungeon, it’s recommended to have a few AOE abilities, and to have more buffs and single target damage abilities to kill the bosses. Learn through some of the bosses and what they do if you do not know how to get out of any tricky situation. With the proper gear, patience, skill, and knowing what to do in certain situations, you will finish all 50 levels in no time!