This has been bugging me for ages ._.

Pretty much at the beginning of the Deep Jungle World you gotta collect some slides to show them to Tarzan. There's one slide which shows a building (most likely a castle?) and Sora says it seems familiar to him, although he never left Destiny Islands before.

 

How does he know that place and what place is it anyway? I was wondering about this since I got Kingdom Hearts 1 back in 2002 and I wasn't able to figure it out until today.

It's the castle in Hollow Bastion because Kairi's heart was inside Sora so her heart reacted to it.

There's two theories, the generally more accepted, it's similar to RG/HB and Kairi recognizes it as her heart is inside Sora which makes Sora think he knows it

And then there's people who say Ven recognizes it as one of the world's we see in BBS (Enchanted Dominion, Castle of Dreams,and Dwarf Woodlands had castles I think)

But I doubt Ven was even thought of when KH1 came out so it's most likely hinting to Kairi,  

As what they mentioned. Kairi's home is in Radiant Garden/ Hollow Bastion. Since her heart is inside Sora, Sora would likely feel that he saw the place before.as for Ventus's theory, I never heard of it, but it could make sense too since the castle looks more into Enchanted Dominion than Radiant Garden...

Because Kairi's heart was in Sora, Sora recognized that place as being something familiar to him. The same thing happens a lot in the game where he would see Kairi or think he knew something because of Kairi's memories. So that castle is basically Hollow Bastion, but who knows when that picture of the castle was taken.

This has been bugging me for ages ._.

Pretty much at the beginning of the Deep Jungle World you gotta collect some slides to show them to Tarzan. There's one slide which shows a building (most likely a castle?) and Sora says it seems familiar to him, although he never left Destiny Islands before.

 

How does he know that place and what place is it anyway? I was wondering about this since I got Kingdom Hearts 1 back in 2002 and I wasn't able to figure it out until today.

It's Cinderella Castle: it's safe to assume the universe Sora lives in is the Disney universe, and the castle is a landmark or something. 

That, or nothing more than a cutesy in~joke,

whichever you prefer. 

I’m sorry but I completely disagree. This has nothing to do with Kairi, the castle in Radient garden looks NOTHING like this slide, but compare it to the building that Roxas and his friends sit on and eat ice cream. Not only the building but the trees surrounding it are a much better fit for the tower in twilight town. And knowing that Roxas is Sora’s nobody so technically he is a part of Sora during the first visit to deep jungle leads us to believe this place exists in someway inside of sora’s heart.