This concept of Tardises is very nice, but I am a bit worried that when they can be used to teleport Kings, achieving checkmate would be a hopeless task. The way they are implemented here In this particular incarnation they are also a bit large for my taste. I would be inclined to make them 2x2 on the outside, and 4x4 on the inside (embedded in an 8x8 board). And have the additional rule that it would not be allowed to move a Tardis when in check. An even more modest implementation would be a single square, and internally 2x2. This has the advantage that every square in it still borders the outside world, so that a King in there can still be easily checked.
This concept of Tardises is very nice, but I am a bit worried that when they can be used to teleport Kings, achieving checkmate would be a hopeless task. The way they are implemented here In this particular incarnation they are also a bit large for my taste. I would be inclined to make them 2x2 on the outside, and 4x4 on the inside (embedded in an 8x8 board). And have the additional rule that it would not be allowed to move a Tardis when in check. An even more modest implementation would be a single square, and internally 2x2. This has the advantage that every square in it still borders the outside world, so that a King in there can still be easily checked.