As for the first picture, I'd say that you may have overdone the size of the source of light a bit. Otherwise it's pretty good in my opinion.
For the second one, as mentioned before you should remove the shimmering portals and replace them with creating portals of your own with glows and other effects. Also, the sky could be improved with throwing some cloud models here and there and changing their tones with perhaps a few glow models.
On with the third picture then. Once again as mentioned before, the quality of the castle's texture looks poor and I'd recommend to build the castle from other models. I can perhaps make and example of it tomorrow and show it to you. I also think that the color of the grass and trees is too bright, atleast the trees should be replaced with others that fit the environment better.
And then the fourth picture. You should avoid increasing the size of a model too much as it will stretch the texture, making it look ugly and with that, perhaps ruining your piece of work. The sky indeed looks good, as others have said before me. But, in my opinion it looks rather much like the end of the world and to avoid this you should probably add hills and such to the horizon, but in this case your sky might be preventing this from being done.
And on the fift picture I don't really have much more to say than what Fladdermasken already said.
Don't hesitate to ask for guidance if you feel like that.