Every vanquished monster gives you gold with which you can purchase upgrades. However, those upgrades only come in the form of attribute upgrades, like attack speed, critical hit chance, etc.
The control interface is not at all intuitive though. For a touchscreen device, one would assume swiping gestures would swing the staff from atop its tower. In trying to do so, I found the thing stuck 90 degrees to normal! Eventually, I discovered that the staff was rotated by tapping on the left or right half of the screen, which in turn swung the staff a discrete amount.
The staff is continually firing, so you don't have to worry about pressing fire. However, those discrete angles means that you have to take more care in lining up your shots as missing is far more likely. Not being able to aim at an arbitrary direction precludes the possibility of the "spray and prey" method of combat!
The monsters come in similarly fixed vectors – i.e. centrally, or to the left or right. Quite often you'll find a full rank, or one on each side. If you can't intercept them before they reach you, they'll stop at the castle walls and begin attacking it. When the wall's health hits zero, it's game over.
Castle Defender is free, so I can't complain too much. However, I felt that the difficulty curve was set too low, and as such the game play became repetitive – I'd have been happy if the hordes of monsters became much denser than they were in the early levels of teh game.