Ok really long but:
Basically, the people who were isekai'd arrive in batches, and the current generation is the 42nd. Every previous cycle ended with their world and empires being devoured by that giant monster. A new world and new generation of kings is made after that.
The Grand Dukes are actually the spirits of the previous kings, reincarnated out of sheer hatred for the monster. Their only wish is to defeat it, and only their current kings can fulfill that wish for them
The Guide started interfering at first because the MC's empire was progressing thru the tower way faster than anyone else before. Even during the MC's hiatus, he remained far ahead of everyone else. No one in previous cycles had ever reached Floor 90 except for them because the world was always destroyed before they could and the previous generations were weaker.
Anyway, so now the Dark Imperial Empire, the MC's empire, absorbed the other empires as vassals, allowing everyone to enter Floor 90 together. The tower has 100 floors, and clearing all of them is the only way to defeat the monster.
From Floors 90 to 99, the tower no longer features normal enemies. Instead, each floor is an implementation scenario where the players relive the fall of a previous empire due to the monster devouring the world. To advance, they have to prevent that empire's destruction. The difficulty is adjusted so they're only fighting a part of the monster and not the real one. Also, each floor is also based on the history of one of the current top 10 empires

Can someone tell me what's going on pleaseeeee I've been reading this for a long time but I don't get it. What is that fingers something in the sky? Why did they came back from the past? Why did that guide have to intervene and stop them from going up in the tower?