As an experiment I tried Onslaught
I bought one tower (the cheepest) and placed it at the center of the screen (on the classic map).
Then I followed a simple rule, I would buy upgrades for the tower as soon as they became affordable, alternating between Damage and Rate, until those were maxed out. Range was upgraded less consistantly - more when I remembered!
I made it to Level 76, which took about 30 minutes
It all felt rather pointless.
Where is the skill? Decision making? Challenge?