I think to add to Rozi's point, it's also likely to be the colours. I think the red and the green do not go, it's a bit too striking, especially with more cartoony textures. The green texture in the first image for those platforms that go off to the right doesn't seem to match the one further back in the image towards the top middle. In the third image, that green wall texture does not work for whole big blocks - I would strongly suggest that the tops of those platforms use a normal platform texture, and there should probably be trim.
In terms of your stone block texture, it does look alright in parts, but the second image really shows that it looks quite mundane with everything else. I would suggest a variety of textures for the fortress, perhaps some more metal-like textures, or simply more of the colours and less of the grey.