Cheers Daz,
It took less than a minute as it all clips together pretty easily. Once it's in a buildable state I'll bring you one to test out if you like to say thanks for the army loan the other weekend, It'll help me out too so you can tell me what needs tweaking.
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This shows the corner problem. Internal corners leave a gap where the two tiles should meet. The only solutions I can think of are to include a square rod that can fit into the corner and runs through the height of the building, or the solution shown below, which is a simple but fiddly rectangle a few mm's big that just sits in the corner.
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Yep there will be different sizes of tiles so buildings can be all sorts of heights. Also they will be able to free stand and not have to be plugged in to a base, meaning you can stack them on top of each other.
Walkways I think I will make so that they clip over the 'lip' of the roof area if that makes sense, then they can be slotted on and taken off at will so as not to get in the way.
Thanks for the feedback so far!