One important thing is that one player in HH (the traitor player) is the attacker (his units coming from orbit and landing on Terra), the other the defender. This is fun as the traitor player can make cool things like orbital bombardements and use drop pods. The defender (imperial player) has other advantages, like being able to win by holding out until the end of the game and being able to produce new units.
The game works with order cards and rewards planning ahead with orders (works with a strategic map where orders can be placed for following "turns"). Each player has its own deck of order cards. Most are different on both sides (examples: pod assault (attacker), vs build titans (defender)).
The combat works with combat cards. Each player has a combat card deck. Each card has special effects if a certain unit type takes part in combat. Cards are used to deal damage and also to ward off damage. Retreats are costly as they force the fleeing army to do nothing for quite a long time (and during that time they cannot retreat again if attacked anew). The order cards decide how many rounds a battle is fought (so some are only minor skirmishes).
Units have ranks which indicate combat strength and damage points. They can be damaged, but mostly not repaired. Some units can be corrupted by the traitor player and then join his side.
The game contains heroes, which allow you to draw additional combat cards if present in battle and have some really game-deciding special rules (mostly only when the unit type they are connected to is present). They can be killed, though this is difficult.
The areas that see most combat are fortress areas which also afford protection and the space ports the traitor player needs to land units on the map. Defence lasers in some areas can shoot at units trying to land.
The game can be won by killing the enemy leader (Horus (traitors) or Emperor (loyalists)) or, after half of the game, by conquering all four space ports. In most scenarios (6 are provided) the imperial player wins if the game ends without one of the aforementioned decisions.
The 6 scenarios provided are not too different from each other, as they are basically different versions of the same battle. But there is sufficient variety, with different event cards used in the scenarios and some special rules.
The game is complex enough for many hours of thrilling battles.