The first two reviews state as fact that this is often called Ibsen's masterpiece, but that is far from the majority critical view, which rightly gives that exalted title to The Wild Duck, with Rosmersholm probably second. However, this is generally considered one of his best plays, though in my view it is the least good of his major works. Ibsen's genius is thankfully such that it still shines through and makes the play worth reading over a century later.
It certainly differs from most of his work, especially his best-known. A late drama, it is usually classified with mostly symbolic later works, in contrast to the "problem plays" that made his name by dealing with contemporary sociopolitical issues. The Master Builder may best be termed psychological and is a very interesting case study of a particular type - a Machiavellian schemer with a guilty conscience. The protagonist is Halvard Solness, a successful aging architect who has gotten his position by ruthlessly and selfishly holding back competition and by an astonishing string of luck he attributes to vaguely metaphysical forces. However, he is deadly certain his luck will soon end, and he fears the younger generation, which only increases his unscrupulous scheming. Worse, he is fully aware of how he has used and held back others and is thus tortured by a guilty conscience. Finally, he has a strange but overwhelming feeling of debt to his wife, whom he believes has suffered as he has gained - first through destruction of her family castle, the rebuilding of which made his career, and the death of their sons. The question of his sanity is present throughout, even to himself, and he often wonders how much, if at all, he is responsible for her trials. He begins to spiral out of control in an ever-widening series of deceits and manipulations, losing the loyalty of both long-time employees and his long-suffering wife, when Hilda, a vivacious young women he knew as a youth, returns. He unsurprisingly develops a great passion for her, and she becomes a double-edged influence - pushing him to literal heights of greatness and daring he had long ceased attempting but eventually driving him to ruin by making him overreach.
Ibsen's usual mastery of dramatic tension is present throughout, and he skillfully keeps suspense high though even less actual action happens than is usual in his plays. What primarily separates The Master Builder from most of Ibsen is the overwhelming sense of unreality, which stands in vivid contrast to the canon of an acknowledged realist master. The character of Hilda is particularly ambiguous; from her improbable entrance on a flimsy, near-unbelievable pretext, her speech and behavior are extremely unnatural. She is interesting because we never quite know what she will do, but the contrast with Ibsen's usual characters is off-putting. Her motivations are unclear, and her effect on others is also very unreal. One can get somewhat used to it, but the strangeness never really fades. We must of course assume this was intentional, though what Ibsen intends is a very open question. She might almost be taken as a dream or figment of Solness' imagination, but everyone notices her and takes her seriously even at her most bizarre. It may be best to see her as a personification of Solness' conscience - or, rather, his anti-conscience, as she pushes him to overcome his scruples. She can thus also easily be seen as a sort of evil Succubus tempter figure, exaggerating his worst qualities by flattering his vanity and soon leading to ruin. Above all, she is another in Ibsen's long line of fascinating, highly nuanced female characters, perhaps most closely resembling the similarly manipulative and ambiguous Hedda Gabler. Other elements are also distinctly unreal, including Solness' constant talk of trolls and demons, which he seems to believe in quite literally. This may be a sign of encroaching madness but is also Ibsen's way of discussing free will, which the play is far from endorsing as real. Some may find all this an exciting change from most of Ibsen; others may think it distracting and/or unevenly executed.
The critical crux is how to interpret the symbolism. Nearly everyone assumes the play is almost fully symbolic, though a few see it as purely psychological, but there are few real suggestions as to what the overarching symbols mean, much less agreement. The tower is an obvious phallic symbol, but there is clearly more to the drama than an old man's lust for a young flirt. Most critics have read the play biographically, seeing it as the mature Ibsen's uneasiness about his canonical position despite having attained great acclaim and fame, specifically his worries about younger writers. However, it certainly goes beyond this, raising issues of the artist's obligation to society vs. creative independence, capitalistic ethics, the artistic conscience, idealism vs. practicality, etc. The ending seems to condemn the darker side of these as leading to bitter doom but is probably the greatest weakness in pure story terms. Ibsen is rightfully known for highly effective, dramatic, and surprising endings, but this - pardon the pun - falls flat. It is predictable, arguably even inevitable; one could plausibly argue Ibsen meant just this, perhaps drawing on Greek tragedy, but it is ineffective, an anti-climax reinforcing already noted weaknesses. The Master Builder still has much to admire, and I would not put it past an enterprising director to bring out its latent genius, but this certainly does not rank with Ibsen's masterworks. I recommend it for fans, but those new to him should being with A Doll's House or The Wild Duck.
