well, they would be robots with a limited range that would stop enemy robots, but if the player got into their range, they would chase and attack the engineer, but you could kil them with projectiles from enemies and towers.
they would have 5 "health" like the player and they would deal 25% of normal tower damage, but there would be 4 per barracks.
they would look something like this:
edit: thanks for replying and sorry for bad grammar
also, nice that you reply to comments