I don't have much experience with mechanic's liens and don't remember the particulars of your case. If you used the architect's plans to improve your apartment (that is, you went ahead with the work and made improvements based on the plans) I think he or she would probably already have a mechanic's lien as to some or all of what you owe him or her, simply by operation of statute without doing anything further. He or she would probably have had to give notice and/or make certain filings to perfect the security interest and/or maintain its priotity vis a vis other creditors, but that's not so much your concern as it only has to do with his or her status vis a vis those other creditors and not with whether you owe him or her the money or he or she has a remedy. I don't think you'd be in a position to sue him or her for punitive damages simply for taking action to perfect the lien, achieve priority, etc. because it's a lien provided for by statute and perfecting, etc. is what the law expects the architect to do. Again, I don't remember the details of your case and it's possible you may have a claim for something else he or she did.
If there is a dispute regarding the amount of the bill (again, I don't remember), that is a different matter from whether or not the architect has a security interest (read "lien"). The security interest goes to whether the debt is secured but has nothing to do with the amount of the debt, which is a contractual matter.
If the architect drew up plans for you but you never went ahead with the work, he or she may not have a security interest at all; I wouldn't think he or she would, in fact, but I"m not sure about this.
A lawyer with experience in these matters should be able to answer these questions off the top of his or her head and not charge you too much to do it. Whether it is worth fighting for $2000 is another question, and it may not be.