You can buy flea bombs for use inside the house, I'd be doing that all through the house (remove all your mattresses and bedding beforehand). I know it's not 'natural' but you really want to get on top of this infestation in one hit or it could go on for months/years.

Then get all your carpets steamcleaned ( you can do it yourself by hiring the machine) to remove/kill all the eggs etc.

Then I'd be speaking to the vet about the best flea product to use...some of the spot-on treatments just kills fleas, others have a residual effect that can kill eggs etc that your pet comes into contact with. Otherwise think about treating the outside areas that your cats hang around in, that might be the source of the problem.