Summary:
I plan to separate the 2:
Forts -->
Protection but no more "Canal" (Ships passing)
Canal -->
"Canal" (Ships passing) but no Protection
Player will have to decide if he really wants to have a "Canal" or a "Fort".
More decisions --> more strategy --> more fun.
By the way:
Canals will not be allowed to become longer than 1 Plot.
(I do not want to create an exploit mechanic for deep inland cities.)
Theoretically you could create however chains like this:
1. City -> 2. Canal --> 3. Lake Plot --> 4. Canal --> 5. Ocean.