I like that castle vault area. The characters stand out well (though the floating one's a bit dark), the colours of the environment suit the setting.
The rest feel a bit... stark? Very high-contrast, to the point that the characters are sometimes difficult to spot, and the colours are very neutral, there's no real sense of atmosphere (this last bit is true of the vault as well). Consider picking a mood colour for each area, and mixing it into all the colours in various amounts. If you're working with a set palette, try to pick colours from a limited part of it for each area.
There are some stray pixels in many of the assets, they could really use a clean-up pass. It's surprising how just taking a few minutes to get rid of single-pixel noise can make a huge difference in the apparent polish of the art.