My question is if there is any possibility to get e.g. two tiles of different tilesets into one area without editing a tileset? When there is, how it works?
And could i use water and castle enemies in one level? Anyway, i never understood why there are different tile sets or sprite sets that prevent you from easily switching between different tiles / sprites...im sure there is a reason but i dont know

P.S. Sorry when there is already a thread like this

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Soory fore mai bed englissh.
aand i Wishh yu aal a habbie nev yeah!

Okay. Would it be easy (i know this should be posted otherwhere) to code a tile set or a sprite set that ,,unlocks'' all? Because im a total beginner and im far too silly for anything like this...
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Soory fore mai bed englissh.
aand i Wishh yu aal a habbie nev yeah!

No, it's not possible.

The reason why "tilesets" exist is that the DS can't have all the tilesets loaded at the same time. It has not enough RAM. So, it only loads the tileset of the current level.

So, we can edit tilesets, but we can't make them bigger because there's no room for them. For the same reason, we can't make a "super tileset" that contains all the tilesets.

Exactly the same for sprite sets

Okay, last idea: A funtion that easily lets you add your own tiles and sprites to a custom pack?
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Soory fore mai bed englissh.
aand i Wishh yu aal a habbie nev yeah!

You have the tileset editor for that. You can make tilesets that contain the tiles you want.

Sprite sets are not editable because that's much, much harder (well, it's impossible). So you're restricted to the sprite set combinations the editor offers.