Civilization The Board Game, next version

Introduction
I deployed a new version of my computer implementation of  Civilization The Board Game. The implementation consists of three parts:

• Civilization Engine 
• Civilization Web Interface
• Demo. Demo version is deployed to Heroku, free quota. Please wait several minutes unless the dyno is brought back to life.

Several new features are implemented: conquering the alien city and victories.  Also, the performance is improved by caching.

Conquering the city and taking the double loot

The alien city can be attacked and pulled down. After winning (or losing) the winner can take the double loot.

According to game rules, the double loot is at the winner's disposal. Some loots are ranked as one, for instance, stealing the trade and some are ranked as two, for instance, the new technology. The number of loot cannot exceed the two.
Military Victory
If an alien capital city is conquered, the victor will get the medal of the Military Victory.

Culture Victory
After reaching the top of the culture track, the Culture Victory is announced.

Economic Victory
When a player puts aside at least 16 golden coins is acclaimed as Economic Winner by the astonished world.

Technology Victory
When a player piles up the stack of technology high enough, the world recognizes him as Steve Jobs and Elon Musk in one person.

Performance improvement
I started to sample the Civilization Engine using VisualVM profiler and immediately discovered the first bottleneck. The game board is every time recreated by executing all commands starting from the initial layout and it was the main resource consumer. The solution was pretty simple, just cache the current board and the latency was reduced even for my free Heroku quota, 1 CPU and 0.5 GB of memory.
Next steps
Unfortunately, it is high time to implement "hanging" feature like "Writing" :
"Cancel a city action being performed by another player (may not cancel resource ability)". I do not have a clear idea of how to accomplish it. One player has to be triggered that another player executes a command eligible for the action and should have a choice: let it or cancel it.