what i think would make it more entertaining would be to make an upgrade system that gives you like 2 options to make more variety in the towers. have like an option to be a sniper tower or a machine gun tower etc.
to do the forcefield staircase you jump to get up the staircase but you will fall through. after that keep jumping while running right then press a and after hit space. the ghost guy will climb up the staircase then unlock the wall