Hmm, wonder why macsen hasn't beaten me to this one...
Cos I only saw it 5 mins before I finished work for the weekend

I'll just add a small note: trade goods are defined for their value in the startpos file. If you want to make some goods really valuable, that's where you'd do it. I think it's the DeclareTradeGoods command, which gives a name (eg WOOL) followed by a value. So if wool is defined at a value of "10" your first level merchant would give 10 florins per year income. If you then build a port and trade it across the seas, you'll get an extra 10 florins for each port that imports it. The second level mechant adds a percentage (I think it's 20% per level, but I could be wrong), in which case your wool becomes worth 12 florins and so on. Now trade incomes are also affected by governor's acumen and other V&Vs so this figure will vary.

And yes, forest clearing is hardcoded as indestructible - making it a very useful item to rename and re-use for something else in mods The Viking port is also indestructible IIRC.

Viking raiding bonus means they recoup the full cost of a destroyed building instead rather than the usual half...