Attempting to build traps upon the walls of the city, after selecting an amount within the limits of available resources, I clicked the build button. After a few second pause, got an "Unknown Error" message and was returned to the screen, No traps, and the iron I had committed had been deducted, but not the other supplies. Tried this a second time, using several of the iron bonuses to rebuild the iron levels, got the same results, Iron gone from trap building but no results, just an 'unknown error' notice.
Any suggestions from anyone? Already out close to 2 million iron, don't feel like keep losing it at these levels.