I don't have much to go on, but I remember reading a young adult fantasy series in which the main character somehow falls from the modern world into a fantasy world. I read this in the mid 1980s but it was not new at the time, so it was probably published in the mid to late 70s.

Things I remember:

• I think it was a series of novels, but it may have been just one.
• I don't think the writing quality was good. It was probably not written by anyone famous.
• There was something about a sword. Maybe you used the sword to travel to the fantasy world.
• There was a big brick or cobblestone plaza that I think was illustrated on the cover and featured in the story ... like the plaza was where you fell into the fantasy world, or something.
• One of the central themes was the protagonist's conflict over staying in the fantasy world, in which I think there was a love interest, or returning home to the familiar modern world.
• At one point I think the protagonist has succeeded in getting passage back to the modern world - performed a service for the grand wizard, or something - which is what he's been trying to do all along. But when he actually gets back, he wants to return to the fantasy world and can't.

Ring any bells for anyone?