I think this is a really good-looking map and I can see how it could be useful for posing and playing in Sandbox.

Although I think the campaign version is also really good, there are a few SLIGHT issues that I think are worth taking note for future reference.

- You can kind of soft lock yourself if you missed the boarded door that you walk past when you go through the combine door. If you don't break the board before going into the Subway tunnel, then you won't be able to open the door from the other side after you blow up the explosive barrels, and you will have no way of going back without no-clipping.

- Some of the invisible walls are kind of weirdly placed and feel a little unnecessary. Specifically, the ones that prevent you from being able to go in between the train when you first enter the subway, and the one that spawns inside that hole in the wall after you kill all the zombies. I think using props like debris or unbreakable boxes would be a better replacement.

- Also while I don't think it was much of an issue in this particular map, I highly recommend adding a text that reads "The End," "You Escaped," or something along those lines whenever you reach the end of a campaign map. If a player isn't aware that they've reached the end of a map, then there's a good chance that they won't know that the map is already finished and might think that they've missed something.

Again, I think this is a very well-made map, and there's a lot of good in here. What I liked the most is how the zombies are able to break through walls, and the level design looks super good! I just felt like offering some suggestions in case the author decides to make more maps like this (which I hope they do)