i have entered through the gate at kvatch, spoken to martin killed the deadra in the coutyard but cannot find the way to the tower. HELP

What does your quest log say?

Isn't there a big friendly arrow on your compass that gently urges you to "go this way"?

Boy, the Wizard is surely getting a lot of visits recently...

I walk throught the firery gate but it does not take me into Oblivion. I just see the gate to Kvatch.

You have to interact with it, not simply walk.
Go near the Oblivion gate and press the interaction button while pointing on its surface.

So in other words

One cannot simply walk into Oblivion.

You'll know if it works when the cursor changes in a small door symbol.

:D

I wonder if any of his guests were rewarded with a brain.

Hi, you may want the game specific forum. https://www.gog.com/forum and search for the game in the bottom left search bar.

This is not the forum you are searching for.

I don't think so, since that would cancel out their immunity to zombies. It's nice to have that sort of thing these days, considering the upcoming Dying Light 2 and all.

Rub the giants knee clockwise two times and anti clockwise thrice. When the giant drops a deuce, rummage through it to find the invisible key. You will know it's the right one by taste.