Loving it, but... – Defender Chronicles - Legend of The Desert King Review

I purchased the mini-Ogre from the Library, and I just now got access to Halflings. When I start a level, both can be built. But as soon as I build one of those 2, the other is no longer available! I really want Halflings giving my Archers Poison arrows along with Ogres stopping the big guys so I can shoot them longer. Is this a bug, or blocked on purpose?
Review by xienwolf on Defender Chronicles - Legend of The Desert King.