Settlement monoliths placed on pyrosphere corners will have a greater sphere of influence, placing monoliths on combinations of 0 and 127 coordinates is a good idea (0:0, 127:127, 127:0 or 0:127).
Leave some room around your manufacturers (at least 1 tile) so that all pods have room to spawn.
The AI cannot walk over water, so having the settlement based on a water side can help.
Don't misunderstand walls in Love. In most games they offer a kind of impassable protection that they do not offer in Love, because the AI can simply walk up them. The main feature of walls is to protect tokens from direct fire or stray shots. Since the 22nd of January, the AI will also build bridges to get over walls.
You can sink placed tokens into the ground using the Smooth_Edit_tool to give them better protection from direct fire.
The caveman's heritage (aka underground settlements)
People love to dig. Underground settlements come in very different design concepts.
Some players place only the monolith underground and some others prefer to place any token that does not need to be powered underground, too.
And then there is the option to power even the manufacture tokens underground.
Drawbacks
Underground settlements should be considered as advanced gameplay feature, because of their buggy nature.
A world tile/block/cell can only be divided/carved out once. The gap has to be filled before you can carve another point.
The AI tends to dig to the underground monolith and/or undermine it, which leads to it's destruction.
Gulliver's Island
Islands are also a great spot to place a settlement. The surrounding water forces the AI to build bridges to reach the settlement.
Good islands for settlement placement should have good access to the surrounding landmasses and are not surrounded by too much high landmasses that would allow the attacking AI a line of sight into the base to shoot tokens.
Eskil's Preference
Eskil stated that he would prefer be a wide, flat area, with a small, tight wall that encloses the tokens to protect them. The flat area helps to spot the approaching AI more easily. Also such a place does not invite the AI do undermine the monolith, like very high places do.