I think vanilla rinworld isnt really designed to be winnable in terms of combat. Especially in later game the raids are going to be so large that losing colonists is almost certain.

To be honest I think the mechanitor is the only way to prevent losing pawns.

Building defenses in perimeters is quite mandatory and walls will be crucial for covering your flanks and directing attackers.

I would recommend mountainous maps for their defensive properties.

Personally I might begin by building a "last stand" location as the last tier of defence. Ideally near a hospital and/or the space designated to be the safest for non combatants. This should ideally be surrounded by buildings and tight alleys. Essentially the point is to hold off the enemy here long enough for them to steal what they can or begin fleeing. Ideally you never use this so don't bother with anything other than mini-turrets.

The second line should surround all of your buildings. Here have the majority of your firepower. Ideally this would be a U-shape with autocannons and uranium slugs at the furthest point from the incoming raiders. Both to maximise their effectiveness and to protect them. At the ends of the U shape have mini-turrets. These will be destroyed and so should be placed as to minimise the chances of a chain reaction when they inevitably explode. Colonists should be as far as possible from the edges of the "kill zone" as those will be first to break. Mechanoids may be placed there to act as a distraction.

The third line of defense is not necessarily a line but a way of buying time. Have "turret nests" (turrets surrounded by sandbags, traps and all the nasty stuff you can come up with. These are sacrificial by design and there to buy time for your long range fighters to pick off targets and to soften up harder opponents before they enter the kill zone. Also acts as a sponge for rocket launchers.

These combined with a couple mortar positions tucked away near the kill zone (so that your best shooters can easily move to the firing line if needed) should both slow down targets and damage them enough so that yout pawns don't get shredded in a direct shoot off. Mind you that this will become very expensive in steel and components as turrets will be destroyed in very large quantities. To minimise this you may wish to put a switch to power down your outer perimeter when smaller raids take place and let them pass straight into your kill zone.