Defense is boring, right? Attacking is action, advance, and victory; defense is static. You sit there and hope for the best. No war was more boring than World War One--unmoving defensive trench lines for four long years. So a priori, you might think a game named "Master of Defense" would be, ah, less than scintillating.
Actually, it's quite cool.
You purchase defensive towers and place them on the map. Initially, you have a choice among three types: ones that attack walking critters, ones that attack flying critters, and ones that attack both but do far less damage. Each tower has arange of attack, which you can see as a circle about it by selecting it.
Then, a bunch of monsters appear at one point on the map and make their way down the road to the point that you're supposed to defend. Any that make it that far kill some number of your 'people'--you have a limited number, and if they all die, you lose. So if at all feasible, you want to kill all the monsters before they get there.
That's only the first wave of attacks; after each wave, you get some additional money, which you can use to improve your towers, increasing their range and allowing them to deal more damage--or to build new ones. Typically, there are dozens of waves for each level of the game. So you see what kind of monsters got through last time, and concentrate on stopping that sort of critter. In other words, while you don't actively attack creatures yourself, the pace of the game is such that you're always actively monitoring what's going on, and planning how to improve your defenses as the attacking waves become more powerful.
Between levels, you gain 'experience points' which you can use to purchase new types of towers, improve the capabilities of existing tower types, and make some other changes, like increasing the amount of money received between waves.
The result is an engaging game where things change enough between each level to retain your interest, and where there's enough for you to do, despite the inherently static nature of being always on the defensive, that you remain engaged.