Book about a kingdom invaded by another world

Question

This is a series my father read when he was younger (he was born in the 60s) and had me read as a kid, the book cover had the look and feel of classic fantasy novels such as the Wheel of Time series.

It is set in a medieval kingdom, the premise of the book was that in the past a portal had opened up to another world and an alien race tried to invade, they were beaten back and the portal was shut.

From what I remember as the series progresses another portal opens up and a war starts with said alien race.

At one point in the series one of the main characters ends up going through the portal and ends up as a slave to an alien race. Where he was enslaved was akin to a plantation in a swamp (I believe).

There is also a very vague memory of female assassin like character in a castle on an peninsula. Full disclosure, that detail may be from a different series/not very helpful.

Answer 1

This sounds a bit like Raymond Feist's "Midkemia" books. The plot described matches at least some elements from the first book(s), Magician (published in two volumes in the US, Magician: Apprentice and Magician: Master), part of the "Riftwar" storyline:

A critical character throughout Feist's series, Pug, is introduced here, and is the focal character for the book.

• The world he lives (Midkemia) in has a culture similar to Earth's medieval period.
• A similar world, Kelewan, exists in another dimension.
• People from both worlds have the ability to use magic.
• A rift is opened between the two worlds, leading to a war between them (hence, "Riftwar").
• Pug has learned magic on Midkemia, but is captured by the Tsurani (from Kelewan), and taken back to their world as a slave.
• He eventually distinguishes himself on Kelewan, and learns their magic as well; he then helps settle the war, and goes home to Midkemia.

Potential points of difference:

• The OP doesn't say anything about magic.
• I'm not sure if this was the first time rifts had been opened between the worlds or not (OP says rifts between the worlds had happened before). There were later encounters with the Tsurani, but I don't recall them as necessarily involving conflict, or enslavement of people from the other world.
• The "alien" race is alien in the sense that they come from another world; however, they basically look human (The Midkemians are usually described as Caucasian; I want to say the Tsurani were more Asian in appearance. In any case, their appearance alone generally didn't seem to betray their origin on another world).