Enmity Campaign is a classic example of the "old school" style of AoS seen around 2004-2006. Hero design was just blooming into a worthwhile pursuit, and most modders were anxious to try new things. Enmity certainly went the distance, with adjustments to the usual formula on almost every front.
The terrain layout has three features of interest. Firstly, the unit production buildings for the side lanes are outside the main base: if they are destroyed, no more units spawn for that lane, and the enemy will march ruthlessly into your main base! How unforgiving...
Secondly, the middle lane produces two smaller waves of minions which run from the mid lane out along the river. They usually head to the side lanes, but every 4th wave they swing back to the mid lane. These serve both as scouting patrols, as backup to the side lanes, and help protect the mid lane against a push (there will always be more troops spawning on the defending side).
Thirdly, one of the most distinctive features of Enmity Campaign are the Mounted Guns. Near almost every tower are 1-3 of these "mini-towers" which players on the defending team can control by placing their hero on a circle near the gun.
If no hero is on the circle, the gun won't fire, and the enemy AI will ignore it. However, enemy human players can attack the guns at any time, so if they're not mounted, they're a valuable asset left completely defenceless.
Thanks to their 1200 range and high damage, mounted guns are an effective means for melee heroes to help defend against pushes and make some money without tanking damage.
Interestingly, the gun layout is the most significant asymmetry on the map. On the bottom lane some of the guns aren't on cliffs, on the mid lane there are twice as many guns, while the top lane is 'normal'.
The game makes full use of Warcraft’s various damage types and armours; some heroes are equipped with piercing attacks, or fortified armour. A 7x7 damage types table is displayed as the map loads, and while it’s a nice thought, I don’t think any AoS needs that level of arbitrary complexity.
The heroes are both memorable and of a good standard for its day. There is a special area in the hero selection screen for the more unique "tech/support" heroes, which include a trap hero, a hero who can produce and upgrade his own mercenary units, and a hero which can turn itself into a copy of another hero.
Some heroes are classified as Mechanical, which renders them immune to certain spells and effects. There is a special shop selling items for Mechanical units, such as motor oil to temporarily improve attack speed, or a repair kit to restore life. Mechanical heroes have exclusive access to the Rocket Launcher item, and land mines placed by mechanical heroes deal bonus damage.
Both towers and mounted guns are mechanical, so it's possible to boost your mounted guns by buying items for them! Mechanical heroes can squeeze extra utility from those items by using them on themselves too.
Enmity Campaign also features a few heroes which are Fliers: they can simply move through the air over cliffs or other obstacles. This is useful for quickly manning guns without taking a detour, or crossing into enemy territory undetected.
Items and Upgrades
Fliers are vulnerable to Nets, a purchasable consumable which pins them to the ground for a short time. A special item called Combat Knife can be used to escape nets, but repeated netting combined with a flyer's typical low health often leads to a kill if they're caught.
There is a Staff of Teleportation available for dirt cheap at 100 gold. It teleports the user to any friendly unit, providing it's still alive after an 8 second channel (4 seconds on the upgraded version). Unfortunately, unmanned guns aren't considered friendlies.
Players may buy Sentry wards, which are invisible, don't expire, and each player can have at most two placed at a time. The wards are crucial for knowing when to defend mounted guns, which offer no vision of their own.
The game imposes a rather arbitrary restriction that heroes cannot carry more than 1 of each item. For example, it's disallowed to hold two of the +2 armour type, but it's fine to have a +2 armour and a +3 armour because they're different items. A handful of items and recipes are team-specific, but this seems to be more for flavour than to create asymmetry.
If you don't want to spend on items, it's also possible to buy Troop Upgrades, such as increased maximum life, movement speed, or damage. The upgrades are quite significant, and neglecting them is a sure way to lose. Regrettably, there isn't any real decision making with upgrades: they all simply make minions "better" rather than specialising them or adding tactics.
Enmity Campaign manages to introduce a very unique map objective in the form of mounted guns, made necessary by the immediacy of defeat upon losing unit production buildings. They interact well with the rest of the game's systems, but in practice can be too difficult to defend against a committed attacker.
The hero variety on a macro level is at least as extreme as most commercial games are today 9 years later: an impressive feat! The balance of the game revolves probably too much around hard counters (nets > fliers, mechanical > magic, damage type weirdness, etc.). Making heroes specialists isn't a problem, but being unable to adapt to the enemy team due to so many immunities is a recipe for frustration in any non-casual setting.