Tap where you want to shoot.

There aren’t many games where you could write a full review on Twitter, and still have enough characters left over to provide a link to an amusing YouTube video of a strange pair of dancing tennis balls. But Ego City Invasion is one of them, and in many respects that’s a credit to the immediacy and simplicity of its gameplay.

You may have heard of Ego City if you’re a social PC gamer, and this is indeed the same virtual city mentioned in the title. The premise, apparently, is that Ego City is under attack, and it’s up to you to repel the invading orc army single handed.

If you do know the PC game, you’ll probably wonder what vehicular warfare against Tolkienian terrorists has to do with anything, and the short answer is: nothing.

But that really doesn’t matter, since Ego City Invasion’s loose affiliation to the developer’s online game is simply a well-planted promotional avenue to entice in the iPhone crowd, which would be unwelcome if the game weren’t so addictive.

It’s a simple landscape, with your city wall on the left and the invading army advancing from the right. Between them is you, in a powerful jet-propelled tank. This armoured vehicle can hover for brief periods, until its engines overheat, allowing you to gain access to a precipice above the battle stage, or to cover ground a little more quickly and avoid any aggressors who slip through.

To stop them, however, you come to the show armed and ready - raining down missiles, bombs, time bombs, nukes and a decent variety of other weapons of medium destruction, as well as being given the option to upgrade your tank as you progress.

Taking out the attacking troops is simply a matter of ensuring there’s clear ground between the two of you and tapping the screen where you want to send a bomb or missile.

To begin with, that seems like about all there is to Ego City Invasion, but a bit of perseverance reveals a lot of superb quirks to the action. For example, the orcs approach out of the mouth of a cave, and sending a volley of missiles into the roof drops a shower of mightily destructive stalactites on their heads.

Trees spring up from the ground at random intervals, which can be a great way to weaken enemies by setting fire to them as the army marches past.

Some zombies are carried by giant birds of prey to be dropped against the vulnerable castle wall, and are armoured enough to withstand a good few bombs. Take out the bird, however, and the fat living dead fall to their second death and take out any colleagues they land on.

Power-ups drop from the sky at times, allowing you to freeze your attackers briefly, or call upon an amusing, angry, heavily-armoured sheep to plough through the advancing troops, killing indiscriminately as it goes. All splendid good fun that reveals some remarkable hidden depths to what initially appears to be an off-the-shelf castle defence game.

That Ego City Invasion simply keeps going, with very little in way of a break between attacking waves, makes for quite an action-packed extravaganza, too, which demonstrates the level of careful, detailed design that’s gone into the game.

We aren’t expecting much in the way of changes, as Ego City Invasion is already well inside Apple’s submissions system, so a very intriguing and spirited strategy shooter should be coming your way shortly.