probability of town portal 
one town (assuming mage guide level 4 is possible)

stronghold    0% 
fortess       0%
dungeon      20%
necropolis   20%
tower        20% without library
tower        29% with library
inferno      20%
castle       20%
rampart      20%

probability of town portal with:
 
2 mage guilds: 1 - ((4/5)^2) = 36%
3 mage guilds: 1 - ((4/5)^3) = 48.8%
4 mage guilds: 1 - ((4/5)^4) = 59%
5 mage guilds: 1 - ((4/5)^5) = 67.2%