I like it, but its hard even on easy map
It gets hard during the early to middle portion where the strongest 1st virus type comes, where you would have just around 200 (when upgrading MGs). Takes out around 30-40 hp. Then nearing the end with the arrow types, they are extremely tough to kill, I died at lvl 49. Can't get enough money and I really don't know what to choose, but it seems that having a lot of high-damage single tower with a lot of slow and stun tower works wonders. Even then, it is still hard, when they get a lot of armor and hp. Maybe giving the in-between pauses more time than 20 secs... my towers don't know what to shoot
P.S. Flamethrower works wonders in midgame, place them in front of the long path rather than a corner for maximum effect.