If you're looking for a cheap games deals on Paradox Interactive titles, Bundle Stars has you covered. They sell games separately, despite their name, and they're running a huge sale including King Arthur and King Arthur II for £2.00 apiece.
Personally I'd recommend both King Arthur and King Arthur II (click here to read my review, though note that the techncial issues have been sorted since), which are available separately or via a £5.00 double pack including DLC. They're slightly odd hybrids of RTS and choose-your-own adventure, with plenty of rough edges yet plenty of heart if you're a fan of the genre. However, I'd also suggest exercising caution as far as Dungeonland is concerned, which is actually free to play on Steam!