No spoilers please! I had the "main idea" of what the Dark Tower is spoiled for me. How much is actually lost?

Was watching a yt video and have it spoiled that the dark tower essentiallyholds multiple universes / realities together.

A bit dissappointed but I was wondering, in what book of the series was this revealed? Would you consider this a major plot point that you're not supposed to know before the end of say, the first book?