What level of harbor to build cogs?

[Dfeng]

As title, thanks

[Gamecock83]

I have achieved level required to construct cogs but can not figure out how to start building process???

[Jeffho]

Level 15 harbor which takes 80hrs! to build making sure Caulking research is done as well. The cog should be available with handcarts and oxen when you select harbor

[USA68]

I have completed requirements to construct cogs but, they don't appear as option to build