Yes, those seams you are getting a common in Unity for some reason when you are using lighting. Very annoying.
The very best to combat that is to re-author your tiles in this kind of way (assuming you have 32x32 pixel tiles):
- Make each tile 34x34 instead (adding a row/column of pixels to each side)
- Have the top row of pixels the same as the pixels that come below it (on the source tile)
- Have the bottom row of pixels the same as the pixels above it
- Have the left column of pixels the same as those on the right
- Have the right column of pixels the same as those on the left
- Change your tileset properties so that there is a margin of 1 pixel around each tile. (This way, your tiles are still 32x32 in the tileset, but 34x34 in the texture)
What you are doing here is making up for the Unity texture sampler going out of bounds due to mathematical precision errors. In this case, should the texture sampler pick a texel “just outside” your tile then it will end up picking the same pixel that comes before it – if that makes sense. (If you look up “seams” and “texture sampling” on the internets you’ll get a better explanation.)
If you have a lot of tiles on a texture you can use a GIMP pluging called Gutter that should automate the process of adding a pixel boundary around each tile.
Here, I’ve found a blog post that explains all this with Tiled, Tiled2Unity, and Gimp: http://www.firebelly.net/post/114529965784/tile-gaps-using-unity-5-and-tiled2unity