Tools are pictured below, just ebay 'hot foam cutter' should bring a large selection up. This initial set cost about 50 quid, quite an expensive outlay however I new I was gonna be doing some major large scale projects so it was worth it for me, heck already saved about £500 on this Fortress ;D
Anyhow, tools from left to right, standard wire cutter, engraver (think this is gonna come into its own when doing the Bio-titan, etc and finally on the right the hot knife. Don't believe anybody when they say size doesn't matter, this thing is awesome, 4.5 inch blade allows you to drill/cut at any angle, a lot more versatile than the bog standard cutter. Also they are mains powered which means no expensive batteries and nice constant power supply

Btw, here the main materials used, poly insulation board, poly ceiling tiles and poly lining paper, about £15 worth in all and plenty left for other projects!

Anyhow, started adding a little detail, still wanting to maintain my cheap n cheerful approach I chanced upon the foam paper liner, about 4 quid for a roll as long as an MP's expense sheet (loooooooooong)
First I made card templates for the commonly used shapes

Transfer these patterns onto the foam and carefully cut out, cuts easy and is very relaxing!

Finally dab of glue on the reverse and lay them down over the structure, this is gonna be one of those lobotomy mind numbing processes which unfortunately there is no way around!

Oh, which brings me to the subject of the glue, I used evostick solvent free impact adhesive, pretty much like the rubber copydex glue but worked out cheaper by volume. Sticks this foam like the proverbial sh*t to an army blanket, most DIY stores stock it
Oh, one more thing, I discovered that the whippy red ruler things GW includes with games would make great columns for the fortress, I only have two if you have any spare please give me a prod, I can paypal postage if your willing to part with them! Cheers