Military strategy at Kleidion

 I ruled out battering rams, as those do not seem to be effective against a barrier shored up behind with earthworks. I compared the range of weapons such as the helepolis or the mangonel or even the ballista with the common compound bow of the time and also considered the use of fire on something made of wood (also considering that the Bulgarians may have thought of this and used animal hides soaked in urine or vinegar to protect against flaming assaults). Mining also seems unlikely. I know that the Bulgarians were holed up in a narrow pass and so have the advantage, but it still doesn't answer questions on how Basil was at first so easily repulsed until he hit upon the idea of flanking them in teh style of the Persians at Thermopylae.  Why could he not have simply launched fire with the use of trebuchet aka helepolis at the Bulgarians and taken down their wooden palisade that way. Why were his losses so heavy? Would the Bulgarians have stayed behind their fortifications or would they have engaged in any hand to hand combat in front? 

There is so little information and I am trying to reconstruct the events as best I can with the knowledge I have of warfare at the time. I am frustrated because I feel there is something I am missing. I'm not even getting into the whole debate of how many soldiers Basil supposedly blinded. That is a separate issue.