What are bonus troops?
Buildings that provide bonus troops can be found in a variety of ways in Samurai Siege. They are commonly rewarded as prizes for competitions and events held in Samurai Siege but you can also purchase them in the Onyx and Ingot Markets with onyx and ingots or by purchasing a promo pack.
Additionally, cashing in 6 unique loot rewards collected in multiplayer battles will give you a Mercenary Camp that gives you bonus Samurai. You can cash in the loot by tapping the castle and then the homeland button.