Although a lot of attention has been paid to develop formulas and methodologies to assess the effect of wind waves on caisson breakwaters, there is still limited understanding of the loadings due to tsunami waves. More specifically, no methodology exists to determine the effect that a partially destroyed armour layer would have on the overall stability of an armoured caisson breakwater. Although Esteban et al. (2008) developed a method to estimate the deformation of a caisson breakwater subjected to solitary wave attack, this method is known the be greatly influenced by the presence of an armour layer in front on the caisson. Thus, laboratory experiments with solitary waves were carried out to clarify the failure mechanism of an armoured caisson breakwater against a comprehensive range a tsunami type events (bore-type, breaking and non-breaking). The results of the experiments show how the load which is exerted by the caisson onto the foundation depends on the depth of water in front of it, the shape of the armour layer and whether the wave strikes the caisson or the armour layer first. Of particular interest, it was found how the presence of armour (in particular a failed or partially constructed armour layer) can sometimes be detrimental to the stability of the caisson itself.