If you were putting the tiles on a surface that was at least 4'x6', then you could probably get away letting gravity keep it all together. However, there's always table bumping and the support surface is often smaller (like a kitchen or folding table), meaning a portion of the tiles need to be able to cantilever off the edge.
They need to be joined somehow, and while I have many ideas about how to do this, the one in the lead at the moment involves clevis pins and hair-pin clips...you may be familiar with them by a different name, so here's what I'm talking about looking under the tiles: (color for clarity)
I'm showing three holes per side in case the tile has a feature that dips below the playing surface, as discussed in the last post and shown here: (pinned at highlighted locations)
Bolts and wing-nuts would provide a more secure connection, but wouldn't be quite as convenient. However, it would be easy to have both on hand and use whichever was more appropriate.
And here's a look at some modular hills placed on the playing surface instead of being joined to the surface:
P.S. The bunkers and vehicles are from 3D Warehouse, my thanks to the original modelers.
See part 1 of this modular table idea here.
See part 1 of this modular table idea here.