Textures collected in https://opengameart.org/content/3d-castle-dungeon-tileset-extended ,
sources below.
Slightly retouched by Michalis Kamburelis:

- To make sizes be power-of-2.

  Although Castle Game Engine can use any texture size,
  but it is best to use power-of-2 size for textures for 3D stuff,
  to enable mipmapping and fast loading at the same time.
  Otherwise, textures have to be resized at loading to enable mipmapping.
  And you want mipmapping in 3D (gives nice look when scaled in the distance).

- To use PNG format for all.

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This is the original notice from https://opengameart.org/content/3d-castle-dungeon-tileset-extended:

released under CC0

used art: (only CC0)

concrete01*.jpg from http://opengameart.org/content/pietextureset

gate.jpg,gate-2.png, ground*.jpg, wall-window.jpg from me

torch model, torch*.jpg from my torch pack (http://opengameart.org/content/fantasy-torch-pack-2)

wall-tex-*.jpg based on wall_breezeblock1.png from http://opengameart.org/content/free-urban-textures-walls-brick-stone-part-1 by scouser

tile_tantiles from BMacZero (http://opengameart.org/node/27679)

shield and sword model/texture from yd (http://opengameart.org/content/shield-sword)

wall-to-depth.jpg made from wall-tex

platform.jpg, made from concrete

mug2.png, model and texture both edited based on my mug pack (http://opengameart.org/content/free-mug-pack-1)

switch.png and decorative.png made by me, used some cc0 textures from scouser and some from me

tile-ground2.jpg from yughues (http://opengameart.org/content/free-tiling-textures-pack-54)