Hey Mick353, welcome to the Forums.
For next time I'd advise you to use the little search function on the top right of the screen. This is a frequently asked question.
This is also the wrong section to ask questions (this is Guild Recruitment for Dinegu).
Whenever you initiate battle with your army, you automatically compete in the weekly Tower Tournaments.
You do need to have the Tower unlocked in your Map, or you will not be awarded possible Victory Medals.
At which Tower you compete depends on the units in your army. The highest Age unit in your army determines which Tower you compete at, despite any number of other units in your army.
Tournaments start around Monday afternoon and end on Sunday afternoon. (CET)
Check the guides section or the FAQ in my signature for more info.
- L