PlayStation Network-exclusive Tower Defence game Savage Moon was great. It was bleak, gory and, above all, hugely satisfying. So it is with great pleasure to find Savage Moon has traversed the murky waters that divide the PS3 and the PSP and arrived on the handheld's stunning screen unscathed.

Subtitled The Hera Campaign, Savage Moon on PSP is pretty much the same game as its PS3 big brother. You need to lay turrets on "Imoons" to defend against increasingly difficult waves of Starship Troopers-inspired aliens called Insectocytes. In typical Tower Defence fashion, there are a variety of turrets to call upon, including machine guns, mortars, anti-air and snipers. But there are a variety of Insectocytes gagging to tear your base to shreds, too, including fast moving critters, flying bugs and even those who fire back.

There are few differences, though, most of which are incidental. The Hera Campaign is set within a gathering of Imoons called the Hera Cluster, which has encroached upon earth space. Of course, that's just a set-up for the 20 new maps the game comes with. There are two new species and the never-ending "vengeance mode", too.

Layered on top of Savage Moon's gruesome innards is an odd Risk-style meta-game called "Tactical Layer". Essentially, it's a level select screen with bells and whistles. Before you take on a level, you're able to micro-manage the resources you want to devote to the attack. New towers become accessible when you complete levels, but you need to be careful, because if you fail - and you will fail - the Insectocytes might retake an Imoon you previously cleared. If they do, you're penalised; the associated unit you gained from clearing the Imoon is lost, and it can only be regained by completing the level again. It's a pain in the arse, to be honest. What you end up doing is powering off your PSP as soon as your base's hit points reduce to zero, and reloading your save game, just to avoid running the risk of losing an Imoon.

Instead, The Hera Campaign's strength lies with the core Tower Defence gameplay. It's great because it's easy to learn but hard to master. Your success depends on careful planning, clever strategy and skill adaptation. The idea is to "funnel" the Insectocytes, which always take the shortest route to your base, into kill zones packed with clusters of turrets that are supported by repair towers. To do that, you need to carefully place blocking towers so that the little gits can't avoid your fire. You also need to be mindful of the type of Insectocyte that's coming next. Anti-air towers, for example, can't fire on ground units. Command Skills, which allow you sacrifice damage output for a boost to cash gain and vice versa, are great for micro-managing the battle; when the Insectocytes are at their weakest it's a good idea to boost cash, then switch to increased damage when the going gets tough. Expert players master the art of "juggling" the Insectocytes by laying blocks then destroying them, sending the creepy aliens back and forth like dogs chasing sticks.

The camera is a little restrictive, but you can zoom in and out with L and R.

Like the PSN game, The Hera Campaign is wonderfully bleak. The music (there are new soundtracks to unlock) is depressing, but in a good way, and the sounds the turrets and the dying Insectocytes make is meaty and terrifying. The graphics, while bland, fit. It's an art style perfectly in keeping with Savage Moon's "all is lost and you're going to die" tone.

And you are going to die. A lot. Played on the easy difficulty, The Hera Campaign is just about manageable, but on the normal difficulty it's brutally hard. We struggled to get past the fourth moon; remember, there are 20. Savage Moon veterans won't be surprised by the difficulty - the original was brutally hard, too - but newcomers might find the steep learning curve too daunting. Thankfully, Bristol-developer FluffyLogic has uploaded a level by level guide to its website, so, if the Insectocytes are beating you bad, help is at hand.

Savage Moon's journey from the DualShock 3 to the PSP has avoided the controls car crash other home to portable transitions sometimes suffer. The nub moves the camera, and pressing X on a clear piece of Imoon brings up the intuitive interface. From there, left and right and up and down flicks of the nub navigate through your units, upgrades, research technologies, Command Skills and other goodies. Then it's just a case of moving the cursor over the desired piece of land and pressing X to build the unit. Simple. Savage Moon's slower pace suits the PSP, as it did the PS3; real time strategy sans a mouse and keyboard is often a recipe for frustration.

Our only criticism of The Hera Campaign mirrors our only criticism of the PS3 game: a lack of multiplayer. Now, some will baulk at the suggestion, but we reckon it would be great to team up with a mate to defend against the Insectocyte horde, whether it be online or wirelessly. Ah well. If we're lucky, FluffyLogic might include the feature in Savage Moon 2, which, fingers crossed, we'll get to see one day.

For now, we'll have to make do with what we've got, which is no bad thing. The Hera Campaign is a wonderful addition to the portable Tower Defence family, and a perfect fit on the PSP. It's hard and depressing, but if you put in the effort, you won't be disappointed.