I saw a closed discussion of this on the Beamdog forum. However, the question was never really answered IMO.
I would say that if solo it should be ASAP. Use thieving skills to the full. Even unlock chests that you will not open. One chest has a gem worth 1000gp.
One thing that slows down going to the Tower in this set-up is that first you have to go to Ulgoth's Beard.
On the way there you can finish the fishermen's quest and Joia's as well if you so desire.
Testing it out, that is what I did. Used stealth to pass the ankheg and agreed to the quest. Also talked to the mother of the adventurer in Ulgoth's.
It was then necessary to make my way to the Tower by way of Beregost. Did a lot of lock picking there to acquire both wealth and experience with 100% safety. Spoke to Marl and Firebead.
In retrospect, it would have been easier to acquire the boots of stealth then.
Got to the salesman. Bought two potions of master thievery and two of perception.
This wiped me out financially.
Sneaked by the guards by going into the shadow of the arch to hide. Because of having only killed one hobgoblin, there were no doppelgangers defending the path to the tower.
On entering went upstairs.
Took a potion of perception and of master thievery.
This enabled finding the traps. Upon opening the chests got many useful potions including a potion of oil of speed.
This was to do more tasks with the previously consumed potions. They are not cheap.
potions
Then race around upstairs opening chests, disarming traps etc. Use the potion from the Candlekeep Inn to avoid getting killed when getting the Tome. Naturally use stealth to avoid combat.
Speak to Riggilo and give him 300gp. After offering to find someone to take Kirilith's hair, give it to him and return to her. I pick-pocketted her before speaking to Riggilo. 
Hadn't been able to buy potion of mirrored eyes, so the basilisks had to wait.
Went to the basement and acquired all the items required by the dwarves. Speed and stealth were all-important. No fighting!
Left the wine in a barrel and left as Fighter L 3, Thief L4. Fighter was almost level 4 too. 300 exp points to go.
Getting to that level in relative safety makes Durlag's a must.
I wasn't able to open the chest with Durlag's Plate in it so took the hit followed by a healing potion.
A true thief wouldn't have been able to take the hit, but would probably have been able to disarm the trap.
I got to that level killing only 1 hobgoblin.
EDIT
Forgot to mention that she had a bag of holding. (EMPTY BEFORE THE START)
Do góry
