Creation of the tiles
Here you can see how I dealt with the tiles until I became familiar with them.
After seeing that it worked well, I went on to the more creative work and developed further the colors and particle system in Blender. (more on this below)
After the grass, I went one step further and started with the wall, which also includes a collision shape.
The last picture here shows the current status (03 January 2020)
I recreated the tiles in Blender, set the camera and animated it so that the camera jumps from tile to tile. So I rendered one tile per frame. In order for the tiles to become seamless, I had to lengthen the shape a bit.
This picture shows grass that was evenly distributed on the tiles using a particle system.
Tileset
I put the individual tiles into a tileset editor together. It was smal editor in the web browser.
I'm not satisfied with this tool and can't even find the link to it again. So I'm looking for a comfortable tool. I would love to hear your suggestions.
Joystick on Touchscreen
It is very important to me that the game works on the smart phone. I definitely had to know whether the controls on the touchscreen is user-friendly.

It worked well with the mouse, but to test it on the touchscreen I had to export it.

It was also a success on the touchscreen. These advancements that I made here are fundamental and important for this project. I was very happy that I succeeded.
Gonkee's tutorial helped me, here is the link.
Mirjam
Mirjam is the sister of Moses and is controlled by the player. Like you see here, I've modeled her. next tasks are the colors / materials and the animations. 
Dialog System
The dialogue system / quest system should play a major role.
Since I am not a good programmer, this is unfortunately very difficult for me.
I look forward to your suggestions and maybe a link or two to a helpful source :)