I suggest you continue to run the adventure as it's described. If the PCs continue to breeze through the opponents, especially if they use surprising or innovative tactics, then so be it. Assuming you're prepared to continue to the next phase of the Adventure Path, there is no harm done. Eventually, the PCs will reach a point where they are challenged. In fact, they may get over-confident and bite off more than they can chew. If you make Blackwall Keep artificially harder, then you will have to keep making the successive chapters that much more harder. If the PCs are the appropriate level for the adventure and have the appropriate quantity and quality of magic items, then all you need to do is play the monsters smart! Have them attack from multiple avenues; perhaps there are tunnels that the lizards can use to sneak into the keep--they're lizards after all, give them burrowing--That way they can enter at multiple points and surround or ambush the party. Have the lizards conduct continual sniping attacks all the time, in order to prevent the PCs from resting--and preventing the wizards from refreshing spells. If the PCs try to ignore these attacks, have the Keep Garrison continually pester the PCs for advice. And then apply penalties to the PCs for being tired! "Barbarian, make a DC 15 Fortitude Save for not getting any sleep or you'll be at -2 for attack rolls etc.," In this manner, the opponents are not getting tougher, but the _conditions_ are!
Another somewhat different tack would be to make the 'conflict' less about the PCs battle and more about the overall defense of the keep. By this, I mean have the garrison pretty much give over running and leading the defense to the PCs. So the PCs need to make some Gather Information rolls to figure out the condition of the ammunition and food reserves, 'Appraise' or knowledge rolls to determine the condition of the walls. Perhaps a Knowledge 'Tactics' (or History or something) to determine the best means of defending the keep...Perhaps Diplomacy rolls or Intimidation to get the troops to the right defensive positions. Handle animal rolls to control the few horses remaining--which happen to go wild at the same time a burrowing lizardman guerrilla sneaks into the stables! The PCs may not have been putting many points into these skills if those skills haven't been meaningfull in play--this way, you're making them meaningful. Then, you can play out the lizardfolk assault on the keep as a simple die roll or two. Make the battle a contest between the 'leader' of the PCs (or whomever the players designate as the character in charge of the defense) and the lizard King. The contest could be between the character's...charisma or knowledge: tactics or whatever skill you think appropriate against the lizard king's Intimidate (if you want to say his lizards fight best when they are afraid of him) or his 'sense motive' (if you want to say that the lizard will win if he can decipherthe keep's defense). The roll is modified by the things the PCs have been doing to ready the defenses: +2 if the Gather Information on the ammunition stores was a success; +2 if the Appraise on the wall was a success (meaning some weak point was identified and repaired); +2 if the Diplomacy to lead the keep's soldiers was a success; and -2 for these rolls if they were a failure.
Of course, you can check out the options for 'Scaling the Adventure'--in general, more is 'better' if you want to take that approach. Perhaps lizards from the Lizardfolk Lair arrive to reinforce the attack on the keep. Forget the Harpy lair, bring the Harpies to the fight! Have them fly over the keep dropping rocks or whatever--or maybe kobolds! Cool! The kobolds won't last long but they're the distraction as the Otyugh sneaks in through the Keep middens.
Finally, if it's just too much like the PCs are killing cannon fodder, then take the lizard king, add the half-dragon template as the 'Scaling' sidebar suggests (or just give him a few fighter levels or whatever you think best) and have him challenge the Barbarian to a one-on-one fight to the death--without weapons--for control of the keep. What barbarian could resist that? And if the barbarian balks, then describe how the Keep garrison thinks he is a coward (and then start deserting).
