A quick re log should fix this also make sure you had enough resources to finish the wall.
Also if you don't mind please post your logs:
This file is saved in your default DOF directory under the name of Log1.txt.
"C:\Program Files (x86)\Steam\steamapps\common\Dawn of Fantasy"
If you are not running the game on Steam, screenshot will be in this folder instead: "C:\Program Files (x86)\Reverie World Studios\Dawn of Fantasy"
In some case you might want to attach all of your available Log files, they are located in the same folder named Log1.txt, Log2.txt, Log3.txt, Log4.txt and Log5.txt,
Thank you for helping us make the game quality better.