I think I am sometimes a victim of changing rules for mixing markdown with HTML...

I've had a several Kata description which looked correct when I made them but months later seem a bit messed up :-(

oh, you're actually talking about the description. Fixed.

Java

in what language?

Excellent kata, enjoyed it very much.
However, I believe that there is an extra whitespace in line 3 of the first "corner" example (shown with a *):

|
|
*\-----

instead of

|
|
\-----

Not an issue.

If this is intended to be a beginner-level kata, perhaps the instructions/requirements could be more explicit about the constructor (order of params?) and getters/setters (method names?). New programmers may not have learned the conventions for getting and setting.