The Recruitment Tavern is a now-obsolete building that was once used to gain extra prizes/rewards for getting new players to try Monster Legends. It has been replaced by the Social - Invite Friends feature.
The building itself is now officially categorized as a decoration. It is a 3x3 unit building that becomes available at player level 5. It sells for 150.
Before being phased out, players would be rewarded with food, gold, and monsters for their inviting their Facebook friends to play Monster Legends.