We have two sliding doors covering the walk-in closet in the entryway and Thommy has a sleeping den in there. If he wants in and the door is closed, he simply slides one to the side and waltzes in.
Another normal closet door he figured out too; the door starts above floor level and all he has to do is stick his paw under its ridge, voilà, open sesame. Luckily the closet bottom is filled with piled shoes, which isn't comfortable as a bed at all, so he sticks to his spots elsewhere.