Player is supposed to prevent horde of creeps from getting to finish point on map. Everyone that make will cost him one life and when there is no life left player lose the game. On the other hand, when there are no more creeps he wins.
Build & Defend is our first game. Since we are only hobbyists in game development industry so far, we wanted something simple to begin with. When we talked about what it should be, we remembered good old times beating levels in classic tower defense games. And it was basically it we decided that our first game will be classic Tower Defense title.
And that "classic" is important in this case. Today you can find lots of TD titles in application stores, but majority is about fixed creep path, tower places, heroes and story. That is not our perception of TD genre.
It should be more about hordes of creeps going straight from start to finish and if there are no towers, that will be put in their path, it will be swift end. It should be also more about competing with your friends and people on-line in race who will make highest score on each map.