Enemies
The headcrab was perhaps the most distinctive enemy in Half-Life, inspiring countless startled jumps as they would hop out of darkened hiding spots in an attempt to eat your brains. Now indeed, they are back in Half-Life 2 to make your travels difficult. What's more, there are now three different kinds of headcrabs to deal with, each with their own special traits.
The old-school headcrab is much the same as it was in Half-Life; they'll creep along in search of prey, and when they get close enough, they'll leap and deal a bit of damage when they impact on you. If you spot them while they're crawling, you can easily take them out with a few shots from your pistol, but they're rarely visible to you before they get into jumping range; they're more often positioned near a corner or the top of a ladder or in some other cramped location, where they'll get a free jump at you before you can react. You can sometimes get a bit of forewarning by listening for their distinctive muttering, but this is rarely audible before they've spotted you.
Luckily, headcrabs do only 5 points of damage, and are easily dispatched, requiring only a few pistol shots or similar. They're most dangerous when encountered in groups of three or four. In this case, more violent means of escape may be required.
First appearing in "We Don't Go to Ravenholm..", the fast Headcrab's name leaves little to the imagination about their speed. They're similar in appearance to normal headcrabs, save for longer legs, and skitter rapidly about the floor, making them difficult to shoot. Besides their increased speed, though, they don't contain any other noticeable upgrades over their less-fleet counterparts; they deal the same amount of damage and can be killed in the same manner, although in this case, the SMG or shotgun are the best weapons to defeat them unlike Headcrabs whom only require well-placed pistol shots.
Poison headcrabs, as their name suggests, carry a poisonous edge to their regular headcrab strikes. First appearing in "We Don't Go to Ravenholm...", they have a loud, distinct screech, not unlike a cat's, when first encountered. Featuring a dark gray mottled back with a meaty maw opening, the Poison Headcrab, if it's attack connects, bring Gordon's health down to 1! While the player's health slower replenishes, 10 points of health are deducted from the total. However, Poison Headcrabs alone cannot kill the player; other enemies striking while in this reduced state can.
Much the same as in the original Half life, Barnacles will latch on to any organic entities that come into contact with their adhesive-coated tongues, and slowly draw their prey up into their maws. This includes Overwatch forces, for the most part, the occasional headcrab, and of course Gordon himself. You shouldn't have too many problems spotting the Barnacles from a distance, though, as their tongues are pretty easy to spot, save in darkened rooms. When you do notice a tongue, killing the beast is a simple matter of popping the Barnacle's main body with eight or nine shots from your pistol; they're immobile, so they can't dodge your fire. A new neat way to dispatch them is the feed them objects with your gravity gun, especially explosive barrels, which if you shoot them as they reach the top, provide spectacular and effective dispatch, especially when there are large groups of them earlier in the game.
Zombies were never a huge threat in Half-Life, but they've been upgraded slightly in Half-Life 2, making for some dangerous enemies. As with the headcrabs, there are a couple of new variants here, corresponding to the new headcrab entries in the bestiary. One common trait between all the zombies is the way that the headcrabs will sometimes detach themselves from the body of a zombie when it's killed. Since zombies often appear in groups, it can be difficult to monitor each individual zombie to make sure that its headcrab dies along with it.
The old-school zombies are slow-moving and fairly weak, and are one of the game's most minor threats. Their only advantage is in numbers, and they'll often swarm you and attempt to surround you before beating you to death with their hands.
Humans who've been converted to the zombie floods by fast headcrabs will take on the abilities of their master. Although they give up some of the pure mass of normal zombies, and appear to be almost spectrally thin, they possess great speed, and are able to close in on you from multiple directions thanks to their great jumping abilities.
Toxic Zombies are among the most physically redoubtable enemies in the game, possessing more pure health than any other bipedal foe you'll come across. On hard mode, they can even take a few crossbow bolts to the chest before going down, when even the Overwatch elites can only take a single bolt before being flung across the room. Unfortunately, they match their defensive capabilities with one of the more potent offensive displays in the game. They don't seem to possess any real offensive attacks of their own except for a swipe from their arms. Instead, they host numerous poison headcrabs on their back. When they spot you, they proceed to grab one and throw it onto you.
Unfortunately, killing a poison zombie is just the beginning of your problems, since, as mentioned, they host numerous headcrabs, all of which will detach from their host when it's killed and start scurrying around, eager to poison you when you get too close. If possible, you're going to want to kill a poison zombie by setting it on fire with the old gravity gun/oxygen canister combination; this will have the beneficial side effect of setting all of the crabs on it on fire as well. It doesn't pay to be a zombie when Freeman is around.
For the bulk of the game, the transhuman forces of the Civil Protection and the Overwatch are going to be your primary nemeses. The Overwatch are somewhat analogous to the Commandos in Half-Life, but are generally smarter and better armed. They usually attack in force, appearing in groups of four or five, especially in the game's open areas, and will attempt to overwhelm you by force of arms.
The Civil Protection forces are essentially cops; they patrol the streets of City 17, looking for civilians to beat, and generally act like thugs, especially when you insist on throwing bottles at their heads. In combat, these guys are only lightly armored, letting you take them down with shots to the body without much effort. They pack both pistols and SMGs.
You'll probably see more Overwatch than you will any other kind of enemies; they're legion throughout most of the last two-thirds of the game. The main difference between the Overwatch and the Civil Protection forces (besides their supremely disinterested and cool radio chatter) are the fact that they boast improved body armor and better weaponry; they'll wield SMGs, pulse rifles, and shotguns in battle, and will quickly become notorious for their grenade skills. They also seem to be able to organize themselves a bit better than the CP troops, and will use squad tactics to gang up on Gordon and attack him all at once from multiple positions.
The high-end Overwatch forces are known only as Overwatch elites, and make rare appearances towards the end of the game. These soldiers are dressed in white, apparently not caring for the way this makes them stand out during urban combat, and exclusively wield pulse rifles. In addition to the normal fire mode, though, they are also authorized to use the pulse rifle's alt-fire, which sends out an unstable core. Luckily, this doesn't automatically disintegrate you, as your own pulse rifle is capable of doing, but will deal heavy damage and knock you around a bit.
Antlions are fairly stupid enemies, but deadly nonetheless, if only because of their numbers. These insect-like creatures are found exclusively near the coastal area of City 17, and will pop up out of the ground when disturbed by any seismic disturbances (i.e. footsteps, a vehicle), and proceed to attack Gordon at close range with their mandibles.
Anlion Leaders differ from their kin in that they are about 4 times the size of a normal ant lion. They have 6 legs, and are just as violent if not more than their underlings. They cannot fly, and rely on running and attempting to run Gordon down and trample him to death. They are decidedly powerful creatures, and can take a lot of damage before they go down. But the rewards are good.
Antlion Guards maintain control over other antlions through the use of their pheromone gland where they emit certain pheromones and noises to relay instructions. Once Gordon slays his first Antlion Guard, he obtains a so called Bugbait and gains control over the antlions for the rest of the game, creating an infinite army of flying troops.
These little drone robots are more annoying than anything, although they do occasionally appear in numbers sufficient to be actually threatening. These little spinning balls of death feature razor-sharp blades, and will attempt to bump into you and whittle down your life; when they approach in large groups, they can also corner you and prevent you from moving. All told, they're very similar to airborne headcrabs.
Combine gunships, whether they're actually piloted or are discrete alien entities, are one of the most challenging foes of Half-Life 2. They'll glide around an area that you're in, constantly stalking the player, and attempt to pound you with their turrets - and, for the record, turrets hurt. To defeat a Gunship the player must use the laser guided rocket of the RPG to steer it past the Gunships defenses and ultimately take it down.
These hulking tanks - which are apparently organic - appear only rarely through most of the game, but show up more frequently in the last level of the game that actually takes place in City 17. As with the gunships, they're armed with a turret with which they'll do most of their hurting, but they also pack some kind of energy beam that can knock through walls and kill you instantly. The energy beam is almost exclusively used in scripted sequences, but can still injure the player in some cases.