The islands are decent, maybe a tad more detail.
The middle isnt that good though.
By looking at it, i can automatically point out the square/ unnatural look to it.
The ground is plain, boring. Add some grass and flowers, maybe even a small tree.
The tower is way to square. I suggest to give it some shape, customize the roof of the tower
The little wooden roof thing is very undetailed, its just poles woth a roof, give it a little "umph" to make it better, just needs more detail, but i dont see how it really fits the map.
The grass also looks very unnatural, if you look in the picture that shows the top of the middle, you will see the ground is very edgy and needs more shape/ a natural look.
Also you should probably make the middle a bit larger, even if that means space the islands out or make more of them.
Im not a builder, but those are some things i would improve on. I think though this map can get a lot better :D
(Im decent at building, just not a pro at it.)(Im a PVP guy :))