'Shiro' means 'castle' in Japanese. I have no idea what 'sama.van' means, but from the context I assume it's something like, 'monsters.attack,' because that's exactly what happens in this game.
Tasked with defending a Japanese castle against hordes of insectoid creatures, you're in charge of building and upgrading defences and standing fast against the ever-increasing numbers of monsters - up to a thousand, on later levels - that swarm over your borders.
You'll also have to effectively manage your resource-collection by building extractor units on top of valuable crystals that jut out of the ground around your castle. Of course, they're often scattered all over the map, so defending your sources of income AND your castle can often prove to be a difficult balancing act, which is very much the point of the game.
In addition to the ravening hordes, you've got weather to contend with: typhoons, earthquakes, and even rain can damage your units (must be all those paper walls), and the only way to defend - if you even can defend - is to ensure that your buildings are fully upgraded.
Considering that sama.van - Shiro is only 69p, there's a lot to like here. The 3D graphics are neat and tidy, if a little uninspired (aside from the central castle, which is lovely but strictly ornamental) and a variety of missions not only teach you the vital mechanics of the game but challenge you with a number of different scenarios, ranging from environmental threats to underpopulation to frantic, haphazard defence.
It's not perfect. The gameplay is a little repetitive, the flat levels leave something to be desired, and the levels go on longer than is ideal in a mobile game - it feels hard to pop in for a quick go and resume later, and I'd prefer to see the overall length of the levels decreased by about a third.
But this is all nitpicking. (If you find the game is dragging a little, there's a handy feature that bumps up the speed - which is handy, if you can react quickly enough.)
Crucially, sama.van - Shiro will set you back less than a quid, which is a steal for this competent, thoughtful, strangely named base-defence title.