Here is a description of the game from Spielbox:
At first glance, Kastell-Castello by Niek Neuwahl looks a lot like Cathedral. It is, however, quite different. The game is played with 2 players. Each player has 7 building elements in the form of different tetrominos which are the be placed onto an 8x8 game board. The player who can place the last building on the board wins.
To make sure the board is variable, there are 3 trees which are freely placeable onto the board before starting. According to the author, that makes for over 4000 different variations. The game also comes with 6 play variants.
Additionally to the regular sized edition, there's also a smaller version of the game which is about half the size of the regular version.
Online Play
Jijbent.nl (Dutch: turn based)
YourTurnMyTurn.com (English: turn based)