See diagram 1:
Modified castling conditions:
1. can castle and check opposing king.
2. king must be next to rook after castling.
white to move:
castles e1-d1;a1-e1 //check
This scenario can apply other files as well.
Can be even used in 8x8 std chess with interesting results.
Strong players will not use b and g file always.
Offensive minded - will castle with rook to the file that poses the biggest threat to enemy.
Free Castling as described gets the best of both worlds by moving the king to corner and moving the rook in the center.
However, I prefer the version I proposed above as in Birds and Ninjas.