I’m a smuggler, and have just completed digging a mile and a half long tunnel from the shore to the city. I don’t know much about magic, but I do know that I don’t want the recent zombie problem to be able to gain access to my tunnel. I hire a Cleric to keep them out permanently, and they choose to cast forbiddance on the tunnel.
Forbiddance states (PHB p.243):
You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor.
The tunnel is 10 feet high, 5 feet wide, and 1.5 miles long, or 7920 feet. This makes its total area 39600 feet, so it would be within the size limitations. Is there any restrictions on how far Forbiddance can affect? Assuming the tunnel is completely unlit, the cleric could only see as far as there darkvision; would the lack of vision have any effect?