Admiral, I agree with you and Gold Bearer that allowing a Crossbowman the same diagonal attack as a Halberdier is broken. I use rule interpretation to handle the problem.
In short, only a Halberdier is allowed to attack diagonally between two squares that block line of sight. Note that a square on the other side of an intervening wall that ends at a doorway is blocked. The light-gray lines represent walls containing an open doorway:
| | | |
| |O| |
|H| B| |
In contrast, a Crossbowman is allowed to attack diagonally in an opponent's corner square if only one of the squares adjacent to the opponent and Crossbowman isn't blocked. So he couldn't attack from the Halberdier's postion above, because the adjacent square in common with the Crossbowman and Orc is blocked by the Barbarian. The Crossbowman may shoot the Orc in the diagram below, using standard line-of-sight rules:
| | | |
| |O| |
|C| | |
The result of this difference in ability means a Halberdier can safely attack from a diagonal space turn after turn, but a Crossbowman will likely get only one unmolested shot from his open position. You can check out
this post and
this post for a more detailed explanation if you're interested.