Now, its one thing to have a nice square terrain tile, its another level up to attach them to each other.
There have been many different methods tried over the years with varying degrees of success.
I've had a look at a few of them and for a variety of reasons they just don't suit me. Now that I've backed myself into a self imposed corner so to speak, my solution is magnets. The smaller ones are commonly used to hold models together. I purchased 100 larger 10mm ones for just under NZ$50.
The trick with magnets is that they have 2 poles (lets just call them + and - as it will make me typing this easier). The trick is to have one of each on a side, and keep this consistent throughout the whole lot. I have made a master side with the longer side to the left. and installed 2 magnets in a + - orientation. As long as the rest match this (or mirror image if you stuff it up like I did) then they will all connect together.
From bottom to top we have the master side, 2 magnets positioned in the right orientation above them (the magnets have a sizable through space interaction to the point that one shattered returning to the stack at high velocity). The magnets are then picked up and placed in the holes. The sides are turned over, placed on a flush flat surface and the magnets forced to sit flat in the hole. Fill the hole up with liquid nails (or similar) and leave till its set.
Assemble the sides round the plastic core and then glue the carpet tiles on top. Job done.
The first 2 completed, and they attach to all edges. The magnets are strong enough for one tile to hang off another but I wouldn't bank my life on it.