The White Castle · Board game rules
Can I go to the training yard without taking an action?
Quick answer
No, you must take the Warrior action during your turn to place a Warrior in a Training Yard.
Details
Placing a Warrior in a Training Yard is explicitly defined as an action within the game rules. The text states, "This action allows you to take the Warrior piece farthest to the left in your personal domain and assign it to any Training Yard" [p.9 — Sending Clan Members to the Court of Himeji].
Actions are performed by placing a die on a specific space that grants that effect. The Warrior action is available at the "Outside the Walls" spaces, where "The game's 3 principal actions are available here: Gardener (page 10), Courtier (page 10), and Warrior (page 9)" [p.7 — OUTSIDE THE WALLS].
You can only perform these actions during your turn by choosing a die from the Bridges and placing it on the corresponding space [p.4 — A Player's Turn]. There is no provision in the rules for moving Warriors to Training Yards outside of this active procedure.
Watch out: You cannot move Warriors to Training Yards at the end of the round or between turns; this must be done as part of your active turn using the Warrior action.