Assuming this is about a residential property, landlords in Pennsylvania are not permitted to keep security deposits without providing a written itemization of costs to the tenant.
If you want to fight this, you'll probably need to go to small claims court. You might consider hiring an attorney to help you, but the cost of that might be more than you'd get out of your deposit, so it's a judgment you're going to have to make for yourself.
This answer does not constitute legal advice and does not establish an attorney-client relationship.
First of all, since you did not have a lease, the landlord does not have a right to collect late fees, unless you agreed to them orally and consistently paid them. Landlords do not have a right under Pennsylvania law to late fees unless they are agreed to in the lease or orally, if there is no lease.
Under Pennsylvania law, a landlord is not permitted to keep any of your security deposit without first sending you a letter no later than 30 days after you have moved out containing an itemized list of the amounts the landlord is withholding from the security deposit for either damages or unpaid rent or late fees. If you have not received this letter and 30 days have passed, you are entitled to go to court to recover all of your security deposit. The law also provides that a landlord is liable to the tenant for twice the amount of the security deposit if the landlord does not comply with the law regarding sending the tenant the 30-day notice with itemized list. You can go to court yourself, or if you don't feel comfortable, you could engage an attorney to do it for you.