Good ways to limit space travel?

[thrash]

Originally Posted by Calvin

The other thing to consider is Lagrange points. I had intended to put [interesting things] at those points. Trojan planets in the L4 and L5 of the stars, ...

These don't exist in a binary system with more-or-less equal masses. You need the smaller mass to be less than ~1/25 of the larger mass for the equilateral points (L4-L5) to be stable. The co-linear Lagrange points (L1-L3) do exist, but stable orbits around them (halo or Lissajous) will be extremely difficult to find and maintain.

I would assume that both systems were colonized from somewhere else at nearly the same time, not one from the other. That gives you a space race to drive development, even if it isn't strictly economical. If your default drive is space sails (photon or magnetic), your "breakthrough" could be the fielding of laser/maser or particle beam boosters.

Your target world could have an optically thick atmosphere that limits attempts to map its surface from the original colony systems.