Once you click on it you can see everything your enemy can. There is not research time. Yes, if you build a castle and have enough resources, you can imperial. For example, building a castle is the same as building a monastery and siege workshop to meet the requirement of two castle age buildings to imperial. The castle adds twenty to your population count, giving you room to build twenty more villagers or units.
Many players forget to get the murder holes technology in the university.
Without this, any type of unit could hack away as long as it wanted to at the base of the castle and not be touched. The Teutons are banned for one reason, that being their faster firing and longer ranged TCs. The smart player will build these TCs in unison with each other so their fire can take down any units challenging them. The Teuton TC prevents early rushes and the longer the game goes the more of the map they control. Don't use trade cogs because they're easily dismissed by enemy ships.