There are two different TD type games.
1) there is the One where you are free to roam the entire map by foot, and the monsters follow a set line. You can get in the way in that one. For each monster past, a you lose a life
2) (snow maze type) My favourite type, the monsters walk through a canyon type path (very small, can only fit maybe 2-3 monsters across) and you build towers and ranged melee units along the edge of the canyons, and same with the 1st, each monster through = 1 lost life.
I prefer the 2nd one. Which one are you thinking about?
I didn't understand the blockage because in snowmaze, you can't build on the monsters path.
Also remember to add heroes, as special units.