Legal Question in Landlord & Tenant Law in Illinois

It is the landlord's property not yours, so it is hardly tresspass. Last I knew it was not illegal to feed a cat.

If the porch is part of your exclusive rented area, you have the right to a reasonable amount of privacy and the landlord should ask to be let in if it's not for some reason like fixing the building, showing the apartment to the next tenant if you plan on moving out, or if there's been some problem that the landlord needs to inspect. But usually porches are outside and although they may be the rear emergency way out of your apartment they are considered a part of the "common area" and the landlord, tenants, and frankly several other people (like utility meter readers) have pretty much free access. As to the cats, if they're your pets you do have a right to control how they are taken care of and as long as the landlord otherwise allows them should respect how you want them fed or not. If they're not yours, then the landlord probably can do what he or she wants, including getting them of of the property. You should check your lease if you have one to see if it says whether the porch is yours alone (most likely not) and whether you're allowed pets like cats.