Help

You can build a bridge on a water sector adjacent to a construction truck.  Just click the sector in which you want to build it and you should see a "create" option.

How many sectors can you cross with bridges?  Could I, for example, bridge an ocean?  Do they block the path of sea vessels trying to cross?

you can build them (slowly) as far as you wish. 4 in a straight line per turn (as you move your construction truck along the new bridge).

They do not inhibit sea travel unless you have troops on the bridge. Other country's troops cannot pass through your troops but everyone can use unoccupied bridges.

IC
Lydia

Yes you can cross an ocean. 

No bridges do not block sea vessels.  However, a land unit on a bridge will attack a ship.  And will prevent units from treatied countries from passing

Can you destroy your own or others' bridges?

Yes, with air or sea units.

i wish contrucks could delete a bridge.