Can a party attack multiple Lts in a single turn?

If so, can they only attack them if they are all on the same overland spot, or can they attack them as they travel?

No, they have to pick them one by one.

So it takes 4-5 game weeks to dislodge a tamalir seige? That sucks for the poor adventurers.

I'm not sure if you can't take them in a row in the same game week - but you never fight more than one at the same time.

No, I know that you can only fight 1 LT each encounter. I'm just unsure whether you can fight multiple Lt encounters in the same week

It's in the handy dandy FAQ available right here on this website.

Q: What happens when there are multiple lieutenants in the same space as the hero party? If the heroes (or the overlord) choose to attack, are both lieutenants present at the same encounter?
A: Each lieutenant is an entire encounter unto himself. If the heroes choose to attack a lieutenant when there are two or more lieutenants in the same space, they must choose one to encounter. The overlord may attack the hero party with one lieutenant at a time. Only if the heroes are still in the same location at the end of the encounter (if the heroes win or the lieutenant flees, or if the space in question is Tamalir) can the next lieutenant attack.

and

Q: If the heroes flee from a lieutenant, may they immediately encounter that lieutenant again?
A: No.

and

Q: If heroes are attacking a lieutenant in a town, can they "visit" the shops, etc. before encountering the lieutenant or only after battles have resolved?
A: Only after battles have ended.

Well and good, but please translate ^^ Does that mean only one lt. per week? And do siege tokens remain when I fight and defeat one of them?

Yes, the heroes may attack several LTs in a row if they are in the same location. The keys are in the FAQ texts above and in the RtL rules.

RtL Rules, page 15:

Total Party Kill
If all heroes are killed during an encounter, the encounter ends and the hero party marker is moved to Tamalir on the Terrinoth map board. The party’s game week action is over , and the overlord has scored a resounding blow!

Party Flight
If at any point during the encounter there are no heroes on the map, but at least one hero fled the encounter (as in “Hero Death and Fleeing,” above), then the party escapes. The heroes receive no loot, but continue their game week action .

So, if the heroes either force the current LT to flee or the heroes themselves flee (at least one hero survives), then they may attack a second (or third, etc) lt. However, if the LT scores a TPK, then the heroes' week is over and they cannot attack again that week.

From the FAQ answers you can see that the heroes may not visit the town between battles to heal, shop, etc. Although heroes that are killed in one battle will start fresh in the next, if there is one. Also, the heroes may not battle the same LT twice in the same week.

As for the siege tokens, as long as one LT remains in the city at the end of the week, all siege tokens remain, no matter who placed them. If at any time there are no LTs in the city, then all siege tokens are removed.

Sounds like it would really be a devastating tactic to place three lts or so in Tamalir...

I thought it was something like that. I just wish the language was simpler and more explicit at times.