I purchased Spellforce Platinum about a week ago and reached the Shiel two days previously. I am stuck. No matter what I do, about 5 minutes after activating the Elf monument, hordes of enemies spawn from two bases (the captured city and the orc camp) and attack incessantly, leading to my inevitable defeat. And when I say hordes, I mean anywhere from a dozen (varies with each attempt) to 20 spawn in each camp every few seconds or so and immediately head for my base. Reading on Steam forums, I discovered that a bug was introduced recently with the updates and others are experiencing a similar (sometimes far worse) problem. However, I do not know if the GOG version received the same updates, so I am not entirely certain mine is bugged or if this is actually normal and I'm doing something terribly wrong.

Hard to say. I definitely remember the enemy on this map being extremely aggressive once they realize you are there. This is actually a mechanic in Spellforce and especially this map. Did the orc scout chain detect you? Einar should have warned you about them. But the numbers still seem a bit high. Are you playing on high difficulty (in the settings)? Then again I remember more a constant stream of units coming at you, rather than bulks. But without footage it is hard to tell.

Sorry I am so late in responding to this. I gave up hope anyone would respond after a few days.

No, this after that when I have already killed them and established my base. (The Elven one) I am playing on the easiest difficulty. This is not like previous maps, where the enemies came at a manageable rate. I can hardly defend my base no matter how many units I have and it is impossible to launch an attack. I have made 4 attempts to do so trying different tactics each time to no avail. Sadly, I have no footage or screenshots, but I could load a save and take a few.

Assuming you haven't run into a bug, if you have a save game from before taking the monuments and if you have a character who can handle themselves on their own (for example Light or Heavy Combat Arts) you can try taking down a few enemy bases on your own before taking the monuments. The AI only starts producing units after you take a monument. You probably won't get them all (especially the main base in the north east) but reducing the number of bases should at least help give you breathing room before "starting".

And just to be absolutely certain: Are you SURE the messengers haven'T escaped. Happened to me as well. Chased the first messenger so far the second saw me and ran as well.

Other than that strategy idea what I can offer you, I recently started making Let's Plays and Spellforce is my first project. I am by now past Shiel and if you want you can watch, maybe it gives some inspiration. I am playing on High Difficulty
https://www.youtube.com/watch?v=RlUJedc9AFc
https://www.youtube.com/watch?v=6c4rZjjx3bE
https://www.youtube.com/watch?v=rf9soPcrP_U
If mine doesn't help there are probalby many others as well.

Just in case it is a bug, do you still have a save from before you first entered the Shiel area? Sometimes that can help reset things. Otherwise probably only cheating helps.