Pentissimo has been removed from the iTunes AppStore due to the cease and desist letter from Mindtwister — The makers of Pentago.
After a pleasant exchange of emails, I agreed to take it down.

Dear Gerry Beggs,
It has come to our attention that you have made an unauthorized use of our copyright, trademark and patents for the product Pentago (the "Work") in the preparation of a work derived therefrom.
We, Mindtwister AB of Sweden and Tomas Floden, have reserved all rights to the Work, first published in 2004. Your work entitled Pentissimo is essentially identical to the Work and clearly used the Work as its basis and openly uses the trade mark Pentago. As below examples show.

Text from your web site
Pentissimo is my implementation of the two-player abstract strategy board game Pentago.

The game is played on a 6x6 board divided into four 3x3 quadrants. Each player takes turns placing a marble of their color (black or white) onto an unoccupied space on the board, then rotating one of the quadrants by 90 degrees either clockwise or counter-clockwise.

A player wins by getting five of their marbles in a vertical, horizontal or diagonal row (either before or after the quadrant rotation in their move).

If all 36 spaces on the board are occupied without a row of five being formed then the game is a draw.

The rules may be simple, but winning can by deceptively difficult. Because you have to rotate quadrants, you need to plan out moves in many different directions. Just when you think you have your winning moves thought out, your opponent rotates a quadrant and suddenly the entire board is different!
As you neither asked for nor received permission to use the Work as the basis for Pentissimo nor to make or distribute copies, including electronic copies, of same, you have infringed upon the rights held by us and could be liable for statutory damages.
I demand that you immediately cease the use and distribution of all infringing works derived from the Work, and all copies, including electronic copies, of same, that you deliver to me, if applicable, all unused, undistributed copies of same, or destroy such copies immediately and that you desist from this or any other infringement of my rights in the future. If I have not received an affirmative response from you by within two weeks from the dating of this letter indicating that you have fully complied with these requirements, we will have to take further action against you.

Very truly yours,

Pål Keller Carlquist
Mindtwister AB
xxxxxxxxxx x-x, Värmdö, Sweden
Tel +46 # ### ### ##, e-mail: ###@###########.se
Pentissimo is my implementation of the two-player abstract strategy board game Pentago.

The game is played on a 6x6 board divided into four 3x3 quadrants. Each player takes turns placing a marble of their color (black or white) onto an unoccupied space on the board, then rotating one of the quadrants by 90 degrees either clockwise or counter-clockwise.

A player wins by getting five of their marbles in a vertical, horizontal or diagonal row (either before or after the quadrant rotation in their move).

If all 36 spaces on the board are occupied without a row of five being formed then the game is a draw.

The rules may be simple, but winning can by deceptively difficult. Because you have to rotate quadrants, you need to plan out moves in many different directions. Just when you think you have your winning moves thought out, your opponent rotates a quadrant and suddenly the entire board is different!
Screenshots
Screenshot 01
Screenshot 03 Screenshot 04