/lit/, I'm looking for a book that I read when I was younger,

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/lit/, I'm looking for a book that I read when I was younger, and naturally I can't remember the title, or even the cover.

All I can remember is the following:
>set in the English Civil War (I think 1645)
>there are some sort of time travellers who use bolt-action rifles in the first chapter to break a siege that had been going for months, in a few minutes (obviously).
>the main character's name was Daniel, but because of the tongue his father (who was one of the time travellers) spoke, it was either Donel or Don'el or something similar.
>it used some sort of science about "power lines" under the ground converging at certain points to allow portals to open between time periods.

I remember reading it and thinking it was a good story, and would like to be able to read it again one day. Can anyone help me?