These look awesome! Two questions:
1) How are they licensed?
2) Where did the background tileset in your first two screenshots come from? Is that avaialble for purchase elsewhere?
The assets (Animated Pixel Monster Pack Level 1 by NamiPixels) can be used for non-commercial or commercial project. Asset may be modified. Modified versions must be plainly marked as such, and must not be misrepresented as being the original. The assets can be used for multiple titles. The asset itself cannot be freely shared or resold!
2) You can find the background tiles here: Pixel Cave Tileset. For similar packs take a look at the creator's page.