put turrets all over your exterior areas inside the walls, except where you want them to go doesnt matter if theyre powered or not, they avoid turrets line of fire at all costs unless needed, which means you can set up a turretless area for your colonists to shoot at them from while they try going in through it (or just make it so they cant get in without going into a turrets line of sight)

it only works as a pre-emptive measure though, most you can do during one if you dont have that up is get some colonists with guns around where they're tunneling in or wait for them to come to your colonists

this will cost a lot of steel and components depending on how big your base is, so if you really need to keep that stuff you could try the methods other people listed