How to counter marine rush as a protoss on maps with huge ramps like Scrap Station?

A bit of a long answer...

tharibo, the MAIN counter is to have A LARGE ENOUGH army. If he brings 10 marines but you have 2 sentries and 2 zealots, you will die no matter what.

If you detected mass marines - don't worry too much. I play mostly Terran (gold/platinum), and my experience tells me that mass marines is a cheese. If you detected that in time (while reactors are being built), then it's easy for you to get ahead in the early game.

Instead of telling you what to do, I will tell you what NOT to do:

1. Don't expand early (should be obvious, but it's still important to mention);
2. Don't be afraid to build assimilators (you will need gas for sentries/stalkers);
3. Don't build 4 gateways (no time and there is no real value in that);

Now, let's talk about what TO do:

1. Build a forge
2. Build 2 cannons on the choke point (not more, don't over-invest; you need to have a mobile army)
3. Get some zealots and put them close to the cannons. Once the marines began to shoot the cannons, your zealots will attack the marines. Often he will lose much more than 3 marines just to kill the 1st cannon
4. If you managed to build a few sentries, put a FF behind the marines so they won't be able to retreat and will die quickly.
5. Once you fight off your opponent, EXPAND! Don't harass your opponent.

Actually, this 'todo' list is very easy, and is simple to modify or extend. After getting 4-6 zealots, add stalkers, etc...

Don't forget to continue scouting your opponent. If you scouted 2 rax (1 tech lab, another reactored) and believe he is going for mass marines, your opponent can easily switch to hellions, etc...

First you need to determine how all-in it is. If he is bring SCVs you should definitely pull probes yourself. A few probes to clog up the ramp can help anyway.

I would NOT get more sentries because they take a long time to build and you need as many real units as possible. Guardian Shield is so good against marines that you should try to get a few though. I usually get 1-3 sentries early so they have time to save energy, and if I scout mass marines than I switch to all zealot stalker. I like about 2-1 stalker-zealot ratio.

Make sure you have your zealots in front.

+1 armor is really good against marines. A cannon can also help if you think they are really going all out 1 base marine push.

Finally, stalkers can kite marines (i.e. they are faster and have more range). So if you see the marines coming send out your stalkers and shoot the marines and then run away as they approach you. If it gets desperate you can continue to do this within your base.

Zealots (w/charge) to Colossi. Zealots take care of marines in a 2-1 ratio. It's a bit scary in the beginning of the game where zealots are fairly "kitable".

If you prefer, trickle in some dark templar instead of colossi. They'll waste scans one by one. I don't even bother with sentries primarily because if I'm heavy with zealots, there is no point to the sentry (guardian shield does not cover them in melee combat from what I understand), and force field generally isn't that effective (unless you are really able to separate their forces) Updated: As long as you've got your zealots within the guardian shield, you'll reduce the damage being done. Sentries do get torn up pretty quickly, so you'll have to weigh that against moving up the tech tree (High Templar, Collossi) against the early battle. Typically, I still find that just having lots of zealots (a token stalker is often good enough to discourage kiting) is enough for the first battle, but you'll have to move onto something more effective. MMM is effective the entire game, so get used to it.

I almost always send three zealots out of a double gate opening to greet my terran friends. Constant pressure will lower the chances you'll be surprised by a mass of marines. (provided you're not running your units against a wall)