A Guardian is reincarnated in his youth and has to defend portals in castles from invading demon hordes

[BFirzen]

The book is about a Guardian/mage, that is reincarnated in his youth/or in a different body several years ago after he is deafeated by demon hordes in the last remaining castle of humanity.
Now he starts over and has to defend portals in castles from invading demon hordes but has the knowledge of his old self.

He has to bind to the portal in his castle.

Read this book sometime in the last 5 years, maybe 3 years ago.

[David]

Hi! Do you remember anything about their powers?

Do you remember details about the cover?

Does the book start with his death and reincarnation or it is something that we got told later?

[BFirzen]

David wrote: "Hi! Do you remember anything about their powers?

Do you remember details about the cover?

Does the book start with his death and reincarnation or it is something that we got told later?"

Hey David,

As it was kindle version, I don't recall the cover.
The death and reincanration of the protagonist was in the first chapter.

The power was something like power to modify the tower / castle and being bound to that castle. Creating stuff and modifying the structure.
If I remeber correctly there was a system that there were many such guardians assigned to castles, and with central management from the kingdom.
Also there was a neighboring kingdom with similar castles but on unfriendly terms.
And there were also some bandit groups, local lords and such.