delivered the opinion of the court.
The only question presented, and upon which a decision of the case rests, is, whether or not the act of the legislature under which the city, by its subsequent action, seeks to enforce the collection of the levy made, is retrospective in its legal sense. It will be observed that this levy is for the purpose of compelling adjacent lot owners to pay the expense of a sewer constructed prior to the time when the act of the general assembly authorizing the action of the municipal authorities, which plaintiffs in error seek to restrain, took effect. By the terms of the city charter in force when the sewer was constructed, it was provided: “ The city council shall cause sewers to be constructed in any district whenever a majority of the property holders resident therein, shall petition therefor, or whenever the board of health recommend the same as necessary for sanitary reasons, and said recommendation is approved by the city council.”
Neither of these conditions was observed, but it is contended by counsel for the city, that it is within the power of the legislature to pass an act curative in its nature, whereby illegal assessments for public improvements may be legalized. *196In the Keese ease, supra, the assessments were declared invalid, not because of any irregularity in making the levy, but for the reason that no obligation to pay for the sewer existed, because the preliminary steps authorizing its construction had not been observed. This omission constituted a conclusive defense against any action on the part of the corporate authorities in attempting to enforce a liability against lot owners for the cost of the construction of such sewer. Before this act no steps could be taken by the city which would create any liability against them on this account, but now by its terms it authorizes the municipal authorities, by the adoption of a resolution nearly ten years subsequent, to reach back, change and modify the effect of a past transaction, and give it a different legal result from that which it had under the law when it took place, by attempting to make legal that which before was void, which renders nugatory the complete defense of plaintiffs and others similarly situated, which, before, they could assert against any action to compel them to pay for this sewer, and authorizes the imposition of a liability with respect to a past transaction which, before, had no existence. This brings the case clearly Avithin the definition of what constitutes a retrospective statute in its legal sense,- which, according to text writers and many well considered cases on the subject, embraces a statute which abrogates an existing right of action or defense, or creates a new obligation oh transactions or considerations already past. Denver, S. P. & P. R. Co. v. Woodward, 4 Colo. 162; Woart v. Winnick, 3 N. H. 473; Clark v. Clark, 10 N. H. 380; The Society, etc., v. Wheeler, 2 Gallison, 104; Hope Mut. Life Ins. Co. v. Flynn, 38 Mo. 483; City of St. Louis v. Clemens, 52 Mo. 133; Sedgwick’s Constr. of S. & C. Law (2d ed.), 160; French v. Deane, 19 Colo. 304.
In the absence of constitutional restrictions, it has been held that it is competent for the legislature to enact retrospective statutes to validate an irregular or defective execution of a power by a municipal corporation, but where the constitution of a state contains a special provision against *197such legislation, such acts, when their legal effect is retrospective, cannot be upheld. Dillon’s Municipal Corporations (6th ed.), §§ 544, 814; Cooley’s Const. Lim. (6th ed.) p. 455. Such laws have always been regarded with distrust, for frequently thereby liabilities were imposed which were oppressive and unjust, the effect of which could not be avoided, upon the ground that they were ex post facto or impaired the obligation of contracts ; and it was to prevent such legislation that several of the states (including Colorado), have an express constitutional inhibition against legislation of this character.
On the admitted facts, the act under which the city authorities claim to derive their power in- this instance, in so far as it affects transactions antedating the time when it took effect, contravenes the fundamental law of the state respecting ' retrospective legislation, and, therefore, renders the attempt of the municipality to enforce any liability as to these transactions, ineffectual. The judgment of the district court is reversed, and the cause remanded, with directions to render judgment in favor of plaintiffs in error, enjoining the defendants from collecting the assessments mentioned in the complaint.
Reversed and remanded.