When we think Metal Slug, our minds turn to classic arcade side-scroller action. What we don’t think of is the tower defense genre. However, that is precisely the catagory which SNK Playmore's new free-to-play title Metal Slug Defense falls into.

That's not to say that there isn't some crossover between the classic games and this new enterprise. Creating a 2D side-scrolling tower defense title has allowed the developer to lift art straight from previous Metal Slug games. The gorgeous, smoothly animated sprites are instantly familiar to fans, while the franchise's library of detailed backgrounds mean that no two levels ever look alike.

With the art done, you would think that SNK Playmore would find the time to do something special with the gameplay. Unfortunately, this isn't the case. The mundane tower defense combat takes place on a single plane and has you as sending unthinking troops directly towards your opponent. With energy timers controlling troop creation, fights quickly degenerate into a waiting game. Your opponent is doing the same thing, of course, so most of your time is spent watching your troops die in the middle of the map.

Upgrades and different unit types can go some way to limiting the bottleneck created by these timers. These units include the machine gunner Marco, who makes short work of armour, and shotgun unit Tarma who is able to decimate the undead. Unfortunately, however you use these units, you are frequently overpowered by sheer weight of numbers. This quickly removes any sense of skill or satisfaction, making the deciding factor whether your army is leveled up enough.

What Metal Slug Defense really wants you to do is invest in medals, which can be used to buy stronger troop types and purchase upgrades. But, with 30 medals costing 69p / 99c and anything of worth costing around 150 medals, this quickly becomes expensive.

In truth, we're a little sad to see the Metal Slug franchise leveraged as a free-to-play grind-fest. While the gameplay is mindless, our nostalgia for the series could have carried us through if the game delivered any sense of reward. Depressingly, however, Metal Slug Defense’s business model overpowers any chance of fun, leaving a pretty, but dull pay-to-win experience.