I have a drop-down pop door. The door itself is the ramp; there is a concrete block at the bottom for the ladies to jump onto and I have small chains to hold it from dropping all the way. I've rigged up a string system to lift those chains out of the way when I close the door, otherwise they get caught between the door and the frame. I can still close it, but not sealed tight. The strings have broken three times and I'm considering other methods, possibly with springs and pulleys.
I have a frame around the opening to keep coop litter inside, although sometimes straw or shavings get shoved over and fall into the lower area when the hens come down in the morning with enthusiasm. I'm happy with my set-up b/c the birds have that space to walk on when they are inside. There is no breeze/rain coming in but it plenty of air circulation. I do lock them in at night for safety and, in the winter, warmth, although I leave it open if we go away for a weekend.
We cut the hole for the door after we'd finished most everything else, even the vinyl floor covering, then used a reciprocating saw to cut the floor out, between the studs (which were spaced on purpose to allow this). (I drilled guide holes from underneath to show where to cut from above.) Then I screwed on cross-pieces for footholds, reattached the ramp/door with a hinge, and then measured how much chain to use on each side and screwed that in. The eye-hooks were measured in place--I ran string up to the ceiling and down again, tied on one side to the middle of the chain and on the other to a washer (to make it easy to grab).
To close it, I crouch outside and pull on the string with one hand (to pull the chains out of the way) while pushing up the door/ramp with the other, then secure it in place with a piece of wood that rotates from a stud on the underside of the coop. Sounds tricky but my 6th grader can do it. You wouldn't need chains if you had a higher base for it to rest against. I would have said that the ramp being off the ground keeps out vermin, but I found a mouse in there last month, so there is limited support for that idea.