Thanks guys, I thought as much. The walls are not that tiled, max 2-3 times on one wall, and they're seamless alright. Perhaps the same tile, but slightly shifted or horizontally mirrored, added and blended by a 'world' PF would help. I'll test that maybe. The final size will be a lot bigger (10.000px wide), so it might not be necessary.
And the horizon is empty because the client needs a top down view of only the city and immediate surroundings. No need to fill it up, maybe later. I was just playing about

Right now my main concern is to fill the city with all these houses (only 3/4 done) and then make the light, fields and surroundings as photo-realistic as possible. I think the 'ditches' and sides of the 'gracht' (canal) need some work, for instance.
Any other suggestions are most welcome!