The limitation on attacking unit deployment is one of those things that sounds nice in theory but could be awful in play.
Suppose the initial four units (none of which have Heroes and none of which have 'big guys', as they all have Tough) fail to secure any objectives on turn one and the defenders play a fast, melee-based army. They could easily send out a unit or two to each objective outside the center of the board, close to within 9" of attacking units (on turn 1 or later) and spend a couple turns shredding them before the automatic 'Alarm' triggering.
The defender has no real reason to trigger the ALARM, and further has every reason not to. Perhaps if you restrict ranged attacks targeting attacking units to a 6+ to hit (ignoring reliable) and automatically triggering ALARM on the first charge (either side) might make this better?
Also, a nitpick - there's a couple places where the term 'Attacker' or 'Defender' is replaced simply with 'you'. For consistency, I'd clean those up.