About the Marston Watchtower..
If you make this move in the right moment, it´s a good investment. I have made it after ending the Aria Quest. At this moment the control of Ikaanos it´s 45 and it´s prosperity it´s 35.
Cons:
An investment of 75.000$ from Fort Serra treasure.
A daily expense of 150$
Pro.
No chance of loosing troops. That´s important. If Control it´s under 40 a 15% daily loss it´s, roughly, a guard per week, so nothing to sneeze at.
Power value of Fort Serra of 150. That´s 300$ of income per day. Plus Accountant and your administration, +480 give or take
Fort Serra Toll. that´s variable, but the last day was 138. Plus Accountants and administration +200
50 influence Daily.
+1% of increase City control up to 60. At this moment 7% dayly.
So, all in all, you gain 550$ daily, so you get your investment back in 140 days. And in this 140 days you get 7000 of influence and increase the control of the city by 10%.
Not bad at all, I must say.