I'll sign up as a mentor. I've got very good English skills, and I've had a background in reading and writing from a very early age. I can critique quite well, having learnt to in a diploma class.
Kattaroten:
-Some punctuation is a little fuzzy. If Eric is referred to as Eric the dwarf, as dwarf is a title in this case, perhaps it should be "Eric the Dwarf".
-The action in the first paragraph is superb. I can see what is happening merely by glancing at key words; and that is most definitely what is needed in an action scene. Well done.
-Some syntax errors ("fear for own safety" should be "fear for HIS own safety") but nothing critical.
-I can see that you've taken the 'splitting ideas into paragraphs' and emphasising a statement by using it in its own paragraph. Well done again.
Second paragraph:
-Minor punctuation errors. Mans should be man's, as he owns the sword.
-The thought not being encased in quotes is fine, often it's a stylistic choice. Some people are irritated by thoughts not being encased in quotes, but others aren't. Me personally? I don't mind.
-I like the way you emphasise the moment "Without warning, without any kind of sound". Repeating an idea in this way is very useful to communicate a character's fear and terror.
-The sentence 'Eric waited what must have been...' is a little clunky. It would work better if there's a break in sentences between 'open the cupboard' and 'the doors swung open with a creak.'
-'He peeked out, nothing...' - the comma could be replaced with a semi colon. Semicolons work like a longer pause with an unspoken 'and' after the semicolon. 'He peeked out; nothing, the corridor was as void of life as the room with the dead man.'
-'...the coast was clear everywhere' is a preonasm. That's when part of a statement is made redundant by the previous part. To say the coast was clear infers that it is completely clear, so 'everywhere' is not needed.
Third paragraph:
-It's unclear who you're referring to with 'His breastplate'. Assume the average reader will not understand. You and I know you're referring to the dead man, not Eric, but this should probably be changed to 'The dead man's breastplate...'
-'Cleft' is an absolutely perfect choice of word. Cleft means 'V-shaped hole', like what a sword cutting partway through a body would be.
-'...him and the floor, there was a lot of blood, more than Eric had ever seen.' This is clunky, but would work well as two sentences. Replace the first comma after floor with a full stop and start a new sentence, so that it becomes 'Blood covered him and the floor. There was a lot of blood, more than Eric had ever seen.'
-Call me crazy, but when Eric considers how much blood is contained in the man it could use a hyphen ( - ) where the comma is. I may be wrong, as I used to over use hyphens.
-Where Eric enters a foetal position, although this part of the paragraph sound awkward when I read it out, in my mind it works fine.
-You forgot a full stop after 'windows.'
-The knocking down of the chair part works well. No comma is needed after 'echoed'.
-The 'no' after the initial one does not need capitalisation. 'No, no, no, no!'
-The little action scene of Eric trying to get back into the cupboard works, however 'Barely being able to close the door before someone burst through the door' is a sentence fragment. It's not clear who is referred to. The correct phrasing would be: 'Eric was barely able to close the cupboard door before someone burst through into the room.'
Fourth paragraph:
-The initial few sentences leading up to the dialogue is fine. I can't find anything wrong with it. Unless you weren't trying to deliberately emphasise the steps between the servant walking to the door and opening it, but otherwise it's fine.
-The dialogue MUST be in 'speech marks' or "quote marks" - either are fine, but you must be consistant in what you use.
-I see you start a new line for each new piece of dialogue. That's good. That helps readers see who is talking.
-The rest of the paragraph is fine, besides the final sentence. That needs to be split into two sentences after 'light of dawn'.
Fifth paragraph:
-It's not clear who is looking for the key in his vest.
-'castles many tilestones' needs to 'castle's many tilestones' as the castle owns the tilestones.
-New sentence starting from 'When he turned the key'.
-Instead of 'thumps was heard' it should be 'thumps were heard'. If something plural is acted upon by a verb, it should be 'were' rather than 'was'. If it was a singular, as in 'A thump was heard', it would be fine.
-As I said, dialogue requires quote marks or speech marks.
-I enjoy the small part of the paragraph where Eric thinks about the Eisenschwärze. There is no fault I can find with it. Well done.
-The simile 'no more sound than a silk rubbing against silk' is very good. I liked it a lot.
-The dialogue is quite good, if a little stiff and formal.
-The remainder of the paragraph is very good. There are minor mistakes in punctuation and grammar, but besides that it's fine.
-'I'm getting jittery by all this fighting' should be 'I'm getting jittery FROM all this fighting.'
-Swore should be sworn.
-In the final part, 'eventually thought,' should be 'eventually though'.
-'Dead tired' doesn't really work in fantasy as it's modern slang. 'Absolutely tired' could work better.
-Start a new sentence from 'his dreams.' Remove 'and'.
I've spoilered a new marked up version for you in a new post.