Actually, I found an interesting tactic and it worked like a charm, after having my b*tt kicked in this mission, I found an alternative option, this way I managed to get ALL guard towers built in 35 days (and if I didn't make as much mess ups as I did, then it would be in even less time).
Thing is, first you need to know where all towers are in the black map at start, at first build a Mages guild (VERY important), warriors, ranger and clerics guild, you may build a rogue's guild as well, since you don't nessecarily need the mages guild for recruitment.
It helps, when you're doing this to put the game at slowest speed since time at this point is of the essence.
Once you've laid down constructions for guild, market, blacksmith (blacksmith isn't even nessecary) then cast your spirit of kings at the 2 towers at the north (near the main minotaur temple, if you don't know where it is, start a game to scout locations and try to memorise the tower locations).
Now, the spirit will deal with them minotaurs, but you can only have him 50% of the time around, so what comes next is extremely important (really!).
Build one mage towers next to each ancient tower, only mage-towers work here since they will be ready the soonest, they may not be enough to kill the minitaurs, but they will keep the minitaurs from killing your workers (and if required, recast the spirit again if things get too rough), while doing this, it is offcource advicable you start clearing out the dens around the map.
the rest of the game mainly is a repeat for each set of towers, if the 2 ancient towers are close together, it will be sufficient to build one mage-tower between them.
It is best you should to them in a certain order though, going from upper left to upper right, to the lower right (since there's just one puny crypt behind that one)
The cool thing about this tactic is that you don't need your heroes to protect the towers, so in turn you can secure your turf more easily and additionally, you may be able to complete the towers BEFORE things get nasty