Tablut is a complete-knowledge, deterministic, and asymmetric board game, which has not been solved nor properly studied yet. In this work, its rules and characteristics are presented, then a study on its complexity is reported. An upper bound to its complexity is found eventually by dividing the state-space of the game into subspaces according to specific conditions. This upper bound is comparable to the one found for Draughts, therefore, it would seem that the open challenge of solving this game requires a considerable computational effort.