de_jean_7777, thanks for the feedback! Yes, I am aware of the problem, but fixing it requires playing with Castle Game Engine internals... It doesn't seem too complex, and Michaelis explained me how-to, but still I'm a little "afraid" to start making the patch... And as it's not too critical I put some more effort in something more interesting, like developing new generation mechanics or tilesets... But I'll surely fix it in some time.
phibermon, thank you! I've been thinking of implementing my own erosion (and extrusion) algorithm... but if there is something available and tested it's perfect! I'll definitely look into it.