I thought that a single level platform would help keep them out because as @PipMenace says, they won't go into a house on a deck. But just having a single level platform actually doesn't work in the same way as a deck and the horse steps up onto it. The only way to stop them is to add a little step to the platform and the horse won't use the step (images attached before and after). I only had the front door locked for horses on my new build because it was on flat ground, but the back of the house is on a platform and Nougat just moseyed right on into the kitchen every morning to say Suly Suly. On one occasion, leaving a little present on the floor for his person while she was eating breakfast. Once the step was added, Simone got the "listen to horse" socialisation on her queue while she was indoors but then it vanished as Nougat couldn't get up the step.
Poor Nougat, he was allowed indoors when he was a foal but now he's just too big!