If the kind of "dirty" you're talking about could be easily
rectified by simply paying a cleaning service $100 or so to
come in and do a good cleaning, then if I were you, I'd do
that and write it off as bad treatment from someone who might
turn out to be not such a great landlord. If a cleaning is
all it'll take to solve the problem, then it's probably not
worth the hassle to get into a scuffle with the landlord about
it.

If you simply can't stomach doing that, or if the condition is
really so much worse than when you did that walk-through that
a simple cleaning won't solve the problem, then I suggest you
research what the regulations are in your locality about the
condition in which a rental property must be delivered to a
new tenant. You didn't mention where you live, so we have no
way of knowing for certain, but it's entirely possible that
the regulations where you live require a property to be
delivered to the tenant in clean condition. Call the local
health department and find out.

Independent of that, you could certainly make the case that
you are entitled to having the property wdelivered to you in
the condition it was in when you inspected it before signing
the lease, but do you really want to go to court over it?

If the rental market in your area favors tenants, i.e., if you
could find another property easily, and if you really want to
play hard-ball with the landlord, you could tell him that
either he cleans the apartment or reimburses you for having it
cleaned, or you will walk, and he can feel free to take you to
court for breaking the lease if he feels like going through a
great deal of hassle and probably losing in the end (since he
broke the lease first by failing to deliver the property in
the condition it was in when the lease was signed). If you do
this, you may end up being taken to court, and you'll
permanently spoil your relationship with the landlord, so you
should think long and hard about whether it's a good idea.