Starting half life map development

In the good old days of 1998 when Half-Life hit the shelves, bundled in the CD with the game is Worldcraft 2.0. It's really primitive and so is the compilers, but it has one very crucial thing: A step by step "In The Beginning..." tutorial that guides you step by step of making a playable, presentable first Half-Life map. The tutorial series was dropped in later Worldcraft versions, and that I think was a misstep. Sure, everyone is borderline terminally online 25 years later but not having an "official" tutorial series leaves new people reaching, bewildered by 25 years of accumulated knowledge, and become paralized.

Luckily for you, the "In The Tutorial" series has been somewhat recreated here on TWHL.
It's good enough but one part deviates from the formerly-official series so it left a basic knowledge gap. Luckier still, I have the old help file, with the original tutorial series, decompiled below: You can take a look at the original tutorials and start your journey the same way I did in the early 00s. Note: the images are not embedded into the text document but you can just cross-reference with the images on the archive.

Welcome!

As for leaks, the map must be completely sealed against the "void".
While it has to do with how the compilers process the geometry and calculate what's supposed to be visible, you can think of it as entities not being allowed to come into contact with the void and so you must build an airtight container for all entities in your map.

It's easy to be tempted to put a giant hollow box around your map to prevent leaks, but this will cause the compiler to think everything in your map is supposed to be on the inside and fail at optimising it. It's essentially turning your entire map into one big open area, which the GoldSrc engine really doesn't like.