nice done! 
20000 :400  = 50 
Means you build 50 walls each side.
I assume you used the maxsize of walls lenght?

How does a new build unit fight, which didn't get a bow, axe, hammer, scythe...?
They avoid fighting and die. :)

Where do you see any flag?

uUh excellent point! That's a hard question!

Maybe I'm missing the interaction between the player-hand and a unit,
like clicking on a mushroom will cause a unit to pics it up.

Maybe I see the well building more like a corn plant and less like a mushroom,

because the player has to invest resources in the first place.

Maybe because of units need a tool (scythe) to harvest corn, it's highly logical that gatering water from a well should also require a tool (~clay pot).

( btw.: Wells should beak down by usage in my opinion.)

A middle way could be a posibility to "force give a tool" to a unit (maybe shown by a red arrow instad of a yellow one).
while the cost of the old tool are considered as lost.

I had this problem some month ago. To solve it I had to reinstall my web browser, now using Firefox Quantum 58.0.2 (64-Bit) Sadly I was unable to point out exactly what dependency was bugging.

you can test that :P  I think catapults and wood sorages will affect nothing, but gates and towers could.

I can confirm that the flag icon for protecting does not show up. but somehow after a while it came.
So you might need to level up more and unlock something else,

Love this!

You can figure out how blackboxes work by thinking :)
build a cycle  and then 100Points* Number of walls.
It doesn't depent on the space inside your walls!