The Spore Launcher also offers quite a powerful choice, firing yellow explosive and corrosive orbs, and reloading unlike anything you've ever seen - you feed the creature/weapon the orbs and it greedily swallows them, waiting for your command to regurgitate and spit.
But the best and most inventive tool is the Barnacle. Just the humble Half-Life regular enemy - the one that sticks to ceilings, lets down its metres-long tongue and then grabs anything it can find to haul up and gobble. But you wear it on your arm! Now it can be fired at anything organic and pull you toward it. If that's an enemy it'll bite them to bits when you get close. And if it's a bit of alien plantlife on the walls, ta-da, you've got a grappling hook!
However, this leads me to a fascinating observation. At various points in the game the elusive and mysterious G-Man shows up. As fans of both main Half-Life games will know, this tends to be behind thick glass panes, just watching. Staring. Waiting. But in one rather fun moment in Op Force, when having to use the Barnacle to get past a giant Gargantuan tied to a bridge, he's stood exposed on another platform. And I fired my Barnacle at him, and it didn't stick! The G-Man is not organic! It's definitely a clue.
Although likely a clue that Opposing Force isn't entirely accurate to the canon. I mean - snort! - at one point you see a picture of Gordon Freeman on a wall marked as "Employee of the Month"! I mean! Snort! This is taking place on Gordon's first day of work! And let me tell you about this mistake in Lord Of The Rings...
There is of course a nice cameo from Dr Freeman himself, just before your first visit to Xen. Your paths cross on your way to a very brief jaunt in the floaty alien terrain, and mercifully once you're there it's substantially simpler than Half-Life's drudging visit. Although, rather intriguingly, it doesn't have to be the only time you visit.
The Displacer, an experimental weapon that lets you fire portals the consume enemies and presumably ditch them back home, has an alt-fire that will send you to Xen whenever you wish. This does mean you might well teleport yourself above a bottomless drop. But it might also deposit you in a brightly coloured oasis of useful bonuses. What a rather brilliant idea that they completely fail to do absolutely anything clever with. It would have been superb if there'd been a few moments where it proved your only successful means of progression, forcing you to experiment with it a bit more, rather than only use when you accidentally click the right mouse.
The game is at its best when it's challenging you to complete a series of tasks. Getting machines working, finding the right pumps and valves, working out how to kill an enormous bug-eyed beast so you can extend a bridge - these moments are not only a ton of fun to play, but they don't patronise you by holding your hand through them. You have to make sure you've headed off in every possible direction and found things for yourself.
A good 10 hours long, 10 years on it's surprising to think of it as an expansion for Half-Life rather than a game in its own right. And accusations that it was just more of the same, as made by some complete lunatics at the time it was released, are silly beyond belief. That could only be said by the sort of person who'd refer to the original game having a wrench in it, instead of a crowbar.
There is no question though that it lacks some of the magic of Half-Life. Often Opposing Force feels like someone accurately followed a recipe, but didn't know the correct temperature to cook it at, or for how long. It drags here and there, and often fails to usefully flag what you should be doing next.
And there's about three too many fights set in large warehouses filled with crates and an unnecessary volume of enemies. It gets forgiven each time by emerging into yet another interesting idea (riding the carts on the rails being a splendid example), but there's still never a moment as exciting as that first time you saw the Marines fighting the Xen from your crawlspace vantage point in the first game.
But let us not forget the grappling Barnacle. Let us focus on what a splendid idea this was, and what a travesty it is that it hasn't appeared in the Half-Life universe since. And nor indeed has poor Corporal Adrian Shephard. After being whisked away by the G-Man, presumably he's now stuck filling in for Xenian geography teachers in another galaxy.
Will you support Eurogamer?
We want to make Eurogamer better, and that means better for our readers - not for algorithms. You can help! Become a supporter of Eurogamer and you can view the site completely ad-free, as well as gaining exclusive access to articles, podcasts and conversations that will bring you closer to the team, the stories, and the games we all love. Subscriptions start at £3.99 / $4.99 per month.