Being able to score a kill on a DG and barely escape with your life isn't what I'd call 'waltzing in for the win'. They still stopped the opponent from reaching the actual goal (unless you're playing Slaughter) of the game.
In the first levels it may be barely. But later on, towers are just a nuisance, als already said.
Besides, wasn't the supposed problem that they were too easy to kill when undefended? Is being able to take a few tower hits an issue now, too?
Yes, that's another problem with towers.
Imho only the damage upgrades for towers need to be balanced. Right now only the first upgrade is useful, the later upgrades will not really make the towers less a nuisance for the enemy and are far too expensive for that. Even if for defending aginst creep hordes those upgrades aren't really worth it (Catapults can tear them down anyway). The damage for each upgrade should be increased and each upgrade should also increase the range of the towers, so that you could theoretically keep up with the enemy Regulus range upgrades, if you have the money.